Sperm production is best when the temperature of the scrotum is one or two degrees Celsius (1.8-3.6 degrees Fahrenheit) below the core body temperature of 37 C (99 F).
Testicle-testing German researchers fitted sensors to the scrotums of 30 healthy men who then sat on a heated car seat for 90 minutes.
After one hour, the average scrotal temperature had risen to 37.3 C (99.5 F), and in one volunteer reached a whopping 39.7 C (103 F).
By comparison, men who sat on unheated car seats reached an average scrotal temperature of only 36.7 C (98 F).
The study, led by Andreas Jung at the University of Giessen, did not verify the volunteer's sperm count or sperm mobility, but the researchers fear that only a slight increase in temperature is enough to damage the sperm-production process, the British weekly says.
Previous work in this field has already found that sitting in a car for more than three hours, even on an unheated seat, can impair a man's ability to conceive.
The report appears in next Saturday's issue of New Scientist. The study appears in full in a specialist journal, Fertility and Sterility.