Britain's only pandas will have one final chance to do the deed this morning as time runs out for them to breed successfully.
Yesterday "natural sparks" flew between Yang Guang and Tian Tian, but the pair failed to mate, as the female bear's limited window of fertility draws to a close today.
Keepers at Edinburgh Zoo opened a "love tunnel" between their enclosures after tests showed Tian Tian had ovulated.
They met five times on Tuesday and "several times" again yesterday but failed to ignite the "spark".
The pandas will have a final chance to mate this morning but keepers are losing hope that Tian Tian will get pregnant this year.
They said yesterday the attempted couplings had been "immeasurable" for the bears because they were both sexually inexperienced.
Iain Valentine, director of research and conservation at the zoo, said: "Each time the pair met we saw a huge amount of eagerness and attraction between Tian Tian and Yang Guang.
"There was lots of vocalisation and encouragement from our female and physical contact between the two. He mounted her several times, however full mating did not occur.
"Although both have bred before and have borne cubs with other pandas, they are both still relatively inexperienced. At the end of the day, this is year one of a 10-year conservation project here at Edinburgh Zoo.
"We are hugely encouraged by how much the natural sparks flew between the two animals, as like humans, not all male and female pandas are attracted to each other. Both were keen to mate but their inexperience showed.
"Baby cubs would have been a bonus this year but we have to appreciate that the pandas have only just arrived and have had limited time to settle.
"Overall, we remain very pleased with the outcome of the last few weeks and it has been a fantastic trial run here at Edinburgh Zoo.
"As animal conservationists and scientists, we have learnt a huge amount in such a short time about this captivating species and we look forward to the next 10 years."
Giant pandas Tian Tian (meaning Sweetie) and Yang Guang (Sunshine) arrived in Scotland from Ya'an reserve in Chengdu, China, on December 4 last year. They went on show to visitors for the first time on 16 December.