George, a nine-year-old De Brazza monkey, knows when it is time for its insulin and is a willing volunteer when its keeper enters its enclosure each morning.
One of eight De Brazzas at Port Lympne Wild Animal Park in Ashford, Kent, the animal leaves its troop and scampers across to welcome its keeper.
"He presents himself at 9am, puts his bottom in the air, and has the insulin," said Tricia Corkhill, a spokesman for the park. "Afterwards he jumps up and wants his food."
Matt Crittenden, the monkey's keeper who is also a diabetic, said: "We have been very lucky because he has trained himself.
"He knows he feels better after the injection so he comes in and presents himself. We give him the injection and he goes again.
"No-one likes being stabbed with a needle but it is an incredibly small needle and does not hurt a bit. It is just annoying."
Mr Crittenden added: "Now he is able to live a normal life, live life to the full, just like a human can."
A second monkey at the park, Nancy, aged 12, is also suffering from diabetes but is not such a willing patient. The Capuchin monkey has to be separated from her siblings to receive her daily insulin.