Mr Ashdown, who is opposed to the legalisation of cannabis, refused to give a direct answer when asked if he had ever taken illegal drugs by a reporter from Oxford University newspaper Cherwell.
Interviewed by David Black in his Westminster office, Mr Ashdown hesitated before replying: "I don't ... sorry ... I do not ... the moment you ask me that question, you immediately have the right to ask anyone else that question, and I do not accept the validity of the question.
"What people do in their privacy is entirely up to them. It's not a matter for journalists."
Just before the interview ended, Mr Ashdown asked to return to the subject and give a more considered response.
"The answer to your question is that what all of us did in some distant moment of our youth is a matter for them, but it's not a matter for politicians today. The moment you ask that question you are entitled to go round and ask other people it," he said.
Today's edition of Cherwell contains only this second answer, but the whole interview was tape-recorded.
Yesterday, Mr Ashdown elaborated further: "What matters is where I stand today - I am totally opposed to the legalisation of drugs."
He said he had visited drug-ridden estates in Peckham, south London, and Moss Side, Manchester: "I suggest that anybody who believes in legalisation should do what I did - go and live there for a few days and you will soon see the effects of drugs that are so corrosive and do damage to our society. Legalisation is not an option."
The issue is divisive within the Liberal Democrats, who voted to set up an inquiry into the decriminalisation of cannabis at their 1994 party conference. The motion was moved by Chris Davies, the MP elected last year in the Littleborough and Saddleworth by-election, who was subjected to sustained attack from Labour for being "high on tax and soft on drugs".
Tony Blair, the Labour leader, has said that he has never used illegal drugs, although he did once add, in a reference to President Clinton's difficulties with the same issue, "if I had, you can be sure I would have inhaled".
A spokesman for the Prime Minister said: "It is an absurd question to ask. As far as we know, the Prime Minister has never been asked it. If he were asked, the answer would be `No'."
David Evans, the robustly populist Tory MP for Welwyn Hatfield, demanded Mr Ashdown's head. "If he has taken illegal drugs - however long ago does not matter - then he should do the honourable thing and resign from the Commons," he said.Reuse content