SUMMARY: Hypophysectomy in cockerels leads to metamorphosis of the seminiferous tubules of the testis involving a massive steatogenesis and production of cholesterol in a manner apparently identical with that occurring annually in wild birds when the interstitial cells become exhausted at the end of the breeding season. A new generation of connective tissue cells (forming the presumptive Leydig cells) arises in the absence of gonadotrophin and becomes sudanophil and cholesterolpositive. This points towards an intrinsic rehabilitative mechanism within the testis, although there is no suggestion that the new generation of Leydig cells could actively secrete in the absence of the adenohypophysis. This post-nuptial tubular metamorphosis is widespread or universal in seasonally breeding birds and occurs prematurely if mating is inhibited by captivity or other unfavourable environmental conditions. Although the described metamorphosis has not been recorded in mammals, hypophysectomized rats underwent tubular changes essentially similar to those that occur naturally (and after hypophysectomy) in birds. At the same time the interstitial cell tissue (which is not renewed seasonally en masse in mammals) did not regenerate after hypophysectomy in rats.