The two pandas at Edinburgh Zoo have been brought together in the hope that they will get on well enough to create some baby pandas.
Yang Guang (Sunshine) and Tian Tian (Sweetie) - have just two days to mate or else Tian Tian's hormones will be on the wane and she will not be eligible for more romance for another 12 months.
Female pandas ovulate just once a year and experts at the zoo had seen signs that their panda was at the right stage to conceive.
Zoo keepers opened a 'love tunnel' between the pair's enclosures but despite being put together five times, for about 15 minutes on each occasion, they have so far failed to mate.
An Edinburgh Zoo spokeswoman said, however, there were "very positive" signs from the meetings.
"Just before each meeting, Tian Tian called out incessantly to Yang Guang and pressed her paws and nose up to the grate separating them," she said.
"Her body language was also very encouraging. Yang Guang has responded promisingly each time.
"Yang Guang showed little aggression during their time together, which included him mounting Tian Tian and panda wrestling.
"Although both pandas have bred before naturally whilst still in China, and both have borne cubs, they are still a relatively inexperienced pair.
"Pandas are only fertile for three days, but it is important to stress we are only at the very early stages of Tian Tian's breeding season."
Keepers were on hand to separate the bears if they fall out instead of falling in love, which is not uncommon.
Sunshine weighs 20 stone and Sweetie is not that much slimmer at 14 stone so they could cause considerable physical damage to one another, if not kept a sharp eye on.
The zoo is very hopeful of future cubs because Sweetie has already given birth in China to twins while Sunshine has fathered a cub.
If the pair fail to get together naturally, the zoo has said it will consider artificial insemination.
The exact time of gestation is not clear but experts say the time from mating to birth ranges from 95 to 160 days.