A device that chills the testicles to improve male fertility has been approved by the Food and Drug Administration.

The testicular hypothermia device -- a sort of athletic supporter attached to a small water pump -- could help as many as American 500,000 couples conceive.

It all started when engineer Andrew Sealfon went to see a doctor about his infertility problem in the late '70s. The ideal testicular temperature for production of semen is about 34 degrees centigrade -- three degrees cooler than body temperature -- and an elevated temperature can render many of the sperm ineffective. Surgery helps sometimes, but "I have this crazy idea," Dr. Adrian W. Zorgniotti told Sealfon.

The idea: Build a device that keeps the testicles slightly damp. Cooling is achieved as the distilled water evaporates.

"It's not the most comfortable thing in the world," Sealfon concedes of the device he designed, which must be worn at all times except at night. He says no one comments on the small pump that hangs from his belt, perhaps because it looks like a beeper.

The device, made by Repro-Med Systems Inc. of Middletown, N.Y., will sell for about $795 and will be available by prescription only.

Footnote on the Sealfon family: It's a girl.