io9 has an excerpt from Emily Anthes's new book, Frankenstein's Cat: Cuddling Up to Biotech's Brave New Beasts, in which she details the experiments:

In the 1960s, the Central Intelligence Agency recruited an unusual field agent: a cat. In an hour-long procedure, a veterinary surgeon transformed the furry feline into an elite spy, implanting a microphone in her ear canal and a small radio transmitter at the base of her skull, and weaving a thin wire antenna into her long gray-and-white fur. This was Operation Acoustic Kitty, a top-secret plan to turn a cat into a living, walking surveillance machine. The leaders of the project hoped that by training the feline to go sit near foreign officials, they could eavesdrop on private conversations.