The UK's pair of giant pandas could be put together today in the hope they will mate.
Bosses at Edinburgh Zoo have said resident pandas Yang Guang (Sunshine) and Tian Tian (Sweetie) are showing signs that they are ready to breed.
Tian Tian, the female, is thought to be in season now, following grumpy behaviour and loss of appetite earlier this month which means "things are about to start to happen".
Experts say she will only have a 36-hour breeding window and during that time the pandas will meet up to three times a day for 15 to 30-minute intervals.
The bears may be put together this afternoon if tests show Tian Tian is ready, the zoo said.
She will also be artificially inseminated to increase her chances of getting pregnant.
Yang Guang is "more than ready" to mate, with zoo chiefs describing him as "a completely different animal" from this time last year when breeding was unsuccessful.
The highly anticipated meeting comes after weeks of monitoring by staff who have been analysing the animals' hormones and behaviour.
Iain Valentine, director of giant pandas for the Royal Zoological Society of Scotland (RZSS) which owns the zoo, said: "The annual panda breeding season is imminent and the next 24 hours are critical. Tian Tian is not quite ready yet; however, her hormones could peak as soon as this afternoon, this evening, overnight or early tomorrow morning. We are now that close.
"Yang Guang is more than ready and is a completely different animal to this time last year.
"We are hopeful as the breeding season of both bears seems to be really in sync this year. However, nothing is a foregone conclusion with giant pandas and with pregnancy in general. Not every successful mating leads to pregnancy."
The zoo's panda-cams have been turned off and the bears will be monitored by Mr Valentine and head panda keeper Alison Maclean, along with Professor Wang from the China Conservation and Research Centre for Giant Pandas.
Mr Valentine said: "The pandas have been communicating heavily through the grate between their outdoor enclosures in the run-up to breeding season. However, when then they first come face to face we will give them a little time to get used to each other before lifting the grilled tunnel door.
"When the door opens the pair will probably wrestle once more to assess each other's strength, which is vitally important for animals of this size and power, and then things should progress from there.
"Artificial insemination will follow on from the natural mating attempts, whether they mate naturally or not."
If Tian Tian becomes pregnant, confirmation will come in mid-July when experts can carry out an ultrasound scan. It would then be likely that her cub or cubs would be born at the end of August or the beginning of September.
The normal breeding season for pandas is mid-April to May.