Minutes later, in his own kitchen, he was fatally shot, apparently by a sniper firing a high-powered rifle from behind his house. Within two hours, Dr Slepian was dead: the latest victim of America's anti-abortion warriors.
According to local reporters, police believe the gunman may have hidden behind the back fence of the Slepians' garden. "Somebody lay in wait," said the local police chief, John Askey.
Dr Slepian, who was 51, was well known locally as a doctor. He was also widely known as one of three doctors in the area who did abortions, and he had been targeted by anti-abortionists for more than a decade. Six years ago, there was jubilation in among anti-abortion campaigners when he closed his office in Amherst during a protest campaign called the Spring of Life, organised by the group Operation Rescue in the Buffalo area.
But Dr Slepian made it clear that he was not closing his abortion practice, just moving it to Buffalo, to remove the inconvenience to the other two doctors with whom he shared premises in Amherst. When the campaign against him began in 1988, he was quoted as saying: "They are not going to scare me. They're not going to threaten me."
He and his family had been openly taunted with shouts of "murderer" from outside their house while they opened presents for the Jewish Hanukkah holiday. One of the protesters sued Dr Slepian after that incident, claiming that he had attacked him with a baseball. The doctor settled the case, paying some of the cost of the man's medical treatment and for damage to his van. The family subsequently moved to the Amherst house, which is well set back from the street.
The combination of this clash and Dr Slepian's unapologetic determination may have made him a prime target. "He was one of the few physicians in this area with the integrity to stand up for what he believed in," said Marilynn Buckham, of another clinic in Buffalo, and she accused "anti- choice extremists" of contributing to an atmosphere of "hate and hostility".
The governor of New York, George Pataki, described the killing as "an act of terrorism" and `a cold-blooded assassination" and said the killer should face the death penalty. Some anti-abortion activists also joined the condemnation. "For anyone to take it upon himself to be judge, jury and executioner is nothing but sheer evil," said a former member of Operation Rescue, Karen Swallow Prior, who is standing on a Right to Life ticket for lieutenant-governor (deputy governor) of New York state.
The Federal Bureau of Investigation and Canadian police had warned only three days before Dr Slepian's death that there was a risk of sniper attacks against doctors who perform abortions in the area either side of the United States-Canadian border. The police even gave out safety hints, recommending that doctors stay away from windows not covered by curtains or blinds and to watch for anything suspicious.
There have been three such sniper attacks on the Canadian side of the border since 1994 and one in New York state almost exactly a year ago, when a doctor was shot at, also in his home, and slightly injured by flying glass. All the attacks have taken place around this time of year.
One theory is that they are related to 11 November, which is Veterans' Day in the US and Remembrance Day in Canada. The date is kept by anti- abortion campaigners as a day of memorial for the dead unborn.
Dr Slepian's name was on a list of "abortion providers" posted on an Internet website, which names doctors said to be "working", "wounded" or "killed". The same site has photographs of aborted foetuses and links to the Army of God, a group that says it bombed one abortion clinic in Atlanta in 1997 and another in Birmingham, Alabama, this year.
The chief suspect in those attacks is Eric Rudolph, who is on the Federal Bureau of Investigation's "Ten most wanted" list. He is also suspected of the bomb attack during the Olympic Games in Atlanta in 1996, and was recently tracked down to North Carolina. However, all attempts to find him have failed, including a weeks-long manhunt in the state's mountainous region. There is no suggestion that Mr Rudolph could have been involved in the latest attack, which is one of a kind with those in the Buffalo region.
President Bill Clinton, who has made no secret during successive election campaigns of his view that abortion is a private matter between a woman and her doctor, said he was "outraged" by Dr Slepian's death and ordered the Justice Department to help catch the killer. "The nation cannot tolerate violence directed at those providing a constitutionally protected medical service," he said. "No matter where we stand on the issue of abortion, all Americans must stand together in condemning this tragic and brutal act."
The immediate condemnation from the New York state anti-abortionist candidate, Ms Prior, is one sign that virulently anti-abortion sentiment may be moving further to the edge of the American political spectrum. The director of Operation Rescue, the Reverend Flip Benham, also said that he did not support the killing, even though Dr Slepian "has murdered countless thousands of innocent children".
While abortion is still noted in opinion polls as one of the few single issues on which Americans would choose who to vote for, candidates for office in next week's mid-term congressional elections who support women's right to abortion are more open and less defensive about their stance than at any time since abortion became a constitutional right in the US 25 years ago. In Canada, abortion has been legal for 10 years. Attendance at anti-abortion protests has dropped, and a number of conservative candidates have encountered public hostility for their anti-abortion views.
So far, no one has been apprehended for the shooting of Dr Barnett Slepian. But, as the police have underlined so often after killings in other contexts, the action of a single gunman with a mission is one of the hardest crimes to prevent.
Abortion-related violence in the US
10 March 1993 Dr David Gunn - the first doctor killed during an anti- abortion protest - shot to death outside Pensacola, Florida, clinic. Michael Griffin sentenced to life sentence.
29 July 1994 Dr John Bayard Britton and escort James Barrett slain outside Pensacola, Florida, abortion clinic. Barrett's wife, June, wounded in the attack. Paul J. Hill, 40, sentenced to death.
30 December 1994 John Salvi shot dead two receptionists and wounded five with a rifle inside two Boston-area abortion clinics. Sentenced to life, he kills himself in prison in 1996.
16 January 1997 Two bomb blasts an hour apart rock Atlanta building containing abortion clinic. Seven people injured. Eric Rudolph charged by federal authorities in October 1998.
29 January 1998 A bomb explodes outside abortion clinic in Birmingham, Alabama. An off-duty police officer is killed, and a nurse critically injured in the first fatal bombing of a US abortion clinic. Rudolph charged but eludes massive manhunt.Reuse content