Catechol-O-methyltransferase (COMT) is a key enzyme in the metabolism of catecholamines. As gossypol, a recent male contraceptive, has been reported to inhibit some respiratory enzymes in vitro, we have studied its effect on rat liver COMT activity. Varying concentrations of gossypol acetic acid were incubated with 3H-methyl-S-adenosyl-methionine, COMT, a different concentrations of norepinephrine. It was found that addition of gossypol into the incubation medium greatly depressed COMT activity, with an I50 of 0.01 mM. A series of inhibitory curves constructed from using different concentrations of the substrate suggested that the inhibition was non-competitive. Addition of blood serum or BSA significantly reduced the inhibitory effect of gossypol. We concluded that gossypol may markedly decrease COMT activity through non-competitive inhibition. Since various organs of the rat have been shown to accumulate gossypol, ingestion of gossypol for the purpose of fertility control may have important effect on the metabolism of catecholamine.