It has previously been demonstrated that passive immunoneutralization of endogenous inhibin results in a dramatic elevation in follicle-stimulating hormone (FSH) secretion in the adult female rat but not in the adult male. The purpose of the present study was to investigate whether the effects of immunoneutralizing endogenous inhibin on FSH secretion in the adult male rat might be masked by the presence of additional, compensating, FSH-suppressing factors. This was determined by examining the individual and combined effects of removing the testicular influences provided by the Leydig cells using the selective toxicant, ethane dimethane sulfonate (EDS), and passive immunoneutralization of endogenous inhibin. Within 24 h of a single i.p. injection of EDS, plasma testosterone levels were lowered to near assay limits and by 3 days were undetectable. Plasma FSH levels were significantly elevated 3 and 7 days after EDS treatment, but not to the levels observed in rats castrated for similar periods of time. Castration of rats, treated 3 days earlier with EDS, resulted in a further significant increase in FSH secretion as compared with EDS-treated, sham-operated controls, indicating that the testes were providing an additional FSH-suppressing factor(s) other than those originating in the Leydig cells. Injection of anti-inhibin serum, into rats treated 3 or 7 days earlier with EDS, induced a further significant increase in FSH secretion that raised plasma FSH to a level comparable to that observed in male rats castrated for similar periods of time. Plasma LH secretion was also dramatically elevated by EDS treatment to levels that equaled or exceeded those observed in similarly timed castrates. Pituitary sensitivity, as tested by the injection of an exogenous challenge of luteinizing hormone-releasing hormone (LHRH), was significantly increased 3 or 7 days after either EDS treatment or castration in terms of LHRH-stimulated LH release, but not in terms of LHRH-stimulated FSH release. Immunoneutralization of endogenous inhibin induced no further observable changes in pituitary sensitivity to LHRH. These results demonstrate that in the absence of the Leydig cells a secondary role is revealed for endogenous inhibin in suppressing FSH secretion that, in combination with the Leydig cell influence(s), accounts for the postcastration increase in FSH. The need to remove the Leydig cell influence(s) to reveal an effect of endogenous inhibin on FSH secretion in the adult male rat may suggest that the inhibin effect is normally masked by the presence of the comparatively larger suppressive influence(s) derived from the Leydig cells.(ABSTRACT TRUNCATED AT 400 WORDS)