The endogenous inhibin content was measured by radioimmunoassay in segments of seminiferous tubules at specific stages of the spermatogenic cycle isolated by transillumination-assisted dissection as described by Parvinen (1982). In addition, the immunoactive inhibin production in response to FSH stimulation was investigated. The inhibin content was greatest in tubules at stages II-VI of the cycle and was lowest at stages VII-VIII. Over 48 h of culture, the secretion of inhibin into the medium was greatest at stages XIII-VI and lowest at stages VII-VIII. The inhibin response to FSH stimulation was greatest at stages IX-XIII and least at stages VII-VIII. The results demonstrate that the secretion of inhibin by adult tubules in vitro varies according to the stage of the seminiferous cycle.