Okadaic acid, a tumor promoter derived from a black sponge, dose-dependently suppressed human chorionic gonadotropin (hCG)-stimulated testosterone production in primary cultured mouse Leydig cells. Okadaic acid at 10 nM caused a 70% reduction in testosterone production, and this inhibition was not due to cell damage. Okadaic acid did not affect basal testosterone production. Our results suggest that okadaic acid may inhibit the hCG-stimulated signaling pathway. We found that okadaic acid suppressed the expression of mRNA for two steroidogenic enzymes, cytochrome P450 cholesterol side-chain cleavage (P450scc) and 3β-hydroxysteroid dehydrogenase (3β-HSD), which catalyze cholesterol conversion to testosterone. It is interesting to analyze the mechanisms by which okadaic acid, which inhibits protein phosphatase 1 and 2A, suppressed steroidogenesis.