The UK's only pandas will come face-to-face later today for only the second time.
The male Yang Guang and female Tian Tian have a window of just 36 hours in which to mate, after tests showed Tian Tian had ovulated, which happens just once a year.
The bears were introduced on Tuesday and, despite being brought together five times throughout the day, failed to mate.
A so-called 'love tunnel' between their enclosure was opened up to allow the bears to get together as zookeepers hoped to speed up the mating process.
There were "encouraging" signs however, the zoo said.
Male panda Yang Guang mounted Tian Tian as the pair wrestled.
Tian Tian "called out incessantly to Yang Guang" and pressed her paws and nose up to the grate separating them.
But any mating will be done in private. Edinburgh Zoo has turned off its 'pandacam' which allows people to watch the animals online.
The pandas arrived in Scotland from Chengdu in China in December last year.
– Iain Valentine, Edinburgh Zoo
We are hopeful of a positive outcome. However, we are realistic that this is only the very first year that Tian Tian and Yang Guang have been in Scotland and that we also have 10 more to go. If Tian Tian does get pregnant, confirmation of this may also not be straightforward. Female pandas often have pseudo-pregnancies where their behaviour and hormones indicate they are pregnant for fairly long periods of time when in fact they're not.
Tian Tian and Yang Guang are the first giant pandas to live in the UK for 17 years.