Edinburgh Zoo said Tian Tian and Yang Guang were "both in great health and things are progressing nicely".
If natural methods fail again artificial insemination will be carried out, with the possibility of using fresh and frozen samples from Yang Guang supplemented with frozen samples from the late Bao Bao, of Berlin Zoo.
Iain Valentine, director of giant pandas at the Royal Zoological Society of Scotland, which runs the zoo, said a female panda in the wild would be mated by several males during the 40 hours she is in peak season in a "multiple-mating strategy" that increases the chances of having a cub.
He said yesterday: "So far, as we would expect, there is no big news regarding changing hormones levels but Tian Tian and Yang Guang are already clearly showing increased interest in one another.
"Similar to last year, alongside our own experts, RZSS is working together with a number of global colleagues on the complex science that goes on behind the scenes.
"Natural mating will be attempted, likely followed by artificial insemination, as recommended by our Chinese colleagues.
"Our preference would be to use fresh and frozen samples from Yang Guang, but there is possibility a sample from Bao Bao will be used, as frozen samples can need supplementation."
Last year experts gave it until April 21 before artificially inseminating Tian Tian for the first time.
It led to hopes she had conceived but weeks later the signs were revealed as a false pregnancy.