The CIA ran a secret Cold War project to implant microphones in cats, declassified CIA memos show.
The documents resurfaced this week after WikiLeaks tweeted a link to the report.
A cat was cut open and a microphone, antenna and battery pack were surgically embedded into its body.
The research was developed to spy on Soviet Russia, and was called 'Project Acoustic Kitty'.
It cost US$13 million (NZ$19 million) over its five-year development in the 1960s.
A transmitter and power supply was sewn into the cat's chest, a microphone was placed inside its ear canal, and a wire was implanted along its spine to use its tail as an antenna.
Former CIA officer Victor Marchetti detailed the horrific experiments in 2001, when the documents were first revealed.
"They slit the cat open, put batteries in him, wired him up," he said.
"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."
Despite their best efforts, the CIA concluded the project wasn't practical - but praised the scientists involved.
"The work done on this problem over the years reflects great credit on the personnel who guided it particularly [redacted] whose energy and imagination could be models for scientific pioneers," the final report says.