The seasonal gonadal steroidogenesis of Monopterus albus in relation to natural sex reversal was studied by in vitro time-course incubations. In vitro steroid metabolisms varied with the developmental stages of the gonad. In the female stage, the major products were either 3 beta-hydroxy-5 alpha-androstan-17-one or unidentified metabolites, depending on the season. In the males, 11-oxotestosterone was the major product during the prespawning, reproductively active period, while in other periods of the seasonal cycle, 5 alpha-reduced metabolites were more abundant. In the intersexes, the steroidogenic pattern changed from a female to a male pattern, i.e., the production of 11-oxotestosterone increased and that of the unidentified metabolites decreased.