Aerobic incubation of rat spermatozoa in a balanced salt solution containing 10 microM of gossypol produced a loss of sperm motility. This effect was not reverted by the presence of 25 mM Theophylline, a well-known stimulant of motility, but was significantly diminished by the addition of 10 mM Mannitol. As mannitol is a superoxide scavenger and we found a close correlation between the loss of sperm motility and the malondialdehyde production, we concluded that gossypol affect rat spermatozoa by producing a peroxidative breakdown of membrane phospholipids.