Inhibin activity in charcoal-extracted spent culture medium of Sertoli cells in Primary culture (CSCCM) was monitored by following the temporal inhibition of serum levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in normal adult male rats. Subcutaneous injection of CSCCM reduced LH and FSH levels within 15 min and maintained the inhibition up to 10 hr. No selective suppression of FSH was observed. The model was used to assess inhibin activity in CSCCM from Sertoli cell cultures incubated with Sa-dihydrotestosterone (DHT), testosterone (T), and estradiol -17p (El) (5 pg/ml) for 24 hr. CSCCM from T and EZ treated groups suppressed gonadotropins while the group exposed to DHT failed to elicit any inhibition. DHT seems to inhibit the release of inhibin by Sertoli Cells.