Adult male rats were subjected to local testicular irradiation, plasma hormone levels and testicular histology being quantified at intervals up to 52 days thereafter. LH and FSH increased coincidently with spermatid but not with spermatocyte or spermatogonia depletion. Testosterone levels seemed to decrease but this effect was not significant. Oestradiol levels showed no significant changes. From the correlations between the various parameters it was concluded that the lack of inhibin was the main cause of the increase in both LH and FSH and that spermatids provide the signal for production of this non-steroidal inhibitor. The site of inhibin production was not definitively established but the results would be consistent with production of inhibin by the Sertoli cells in association with spermatids.