Thioproline, which is readily nitrosated to form nitrosothioproline, is expected to act as a nitrite scavenger. The effect of thioproline as an inhibitor of the carcinogenesis induced by N-nitroso-N-benzylmethylamine precursors was examined. Two groups of male F-344 rats were given diet containing 0.25% N-benzylmethylamine (group I) or 0.25% N-benzylmethylamine plus thioproline (0.25% until wk 17 and then 0.5%; group II). Both groups were given drinking-water containing sodium nitrite (0.1% until wk 17 and then 0.2%). The experiment was continued for 717 days. Squamous cell carcinoma of the forestomach developed in six out of seven rats in group I and in significantly fewer, two out of nine rats, in group II. The degree of invasion by the tumours was also less in group II rats, given thioproline, than in group I. Thus thioproline suppressed carcinogenesis induced by N-benzylmethylamine and nitrite, possibly by inhibiting the in vivo nitrosation of N-benzylmethylamine.