Recently partially declassified documents reveal that the CIA succeeded in training a cat. This was such a breakthrough that the researchers were heavily praised for their scientific achievement. However, after one real world case they decided it wasn't practical.

In a project known as “Acoustic Kitty” the Directorate of Science and Technology sought to train a surgically altered cat, wired with transmitting and control devices, to become a mobile, eavesdropping platform. In its first test, the cat was run over by a taxi. According to Victor Marchetti:

they slit the cat open, put batteries in him, wired him up. The tail was used as an antenna. They made a monstrosity. They tested him and tested him. They found he would walk off the job when he got hungry, so they put another wire in to override that. Finally, they’re ready. They took it out to a park bench and said “Listen to those two guys. Don’t listen to anything else – not the birds, no cat or dog – just those two guys!” ... They put him out of the van, and a taxi comes and runs him over. There they were, sitting in the van with all those dials, and the cat was dead!

Science, Technology and the CIA Scroll to Document 27.

This was mentioned in Dave Barry's new blog.