SUMMARY: The pars distalis of the pituitary, the testes, thumb pads, seminal vesicles and thyroids were studied in common frogs (Rana temporaria) kept at 20–24° c for 4, 6 and 8 weeks respectively from 11 October. They were compared with those of control animals kept at normal outdoor temperatures during the same periods. The following effects of high temperatures were noted: (1) In the distal lobe of the pituitary a regression of the γ-cells and increased secretory activity of the β-cells. (2) In the testes formation of cholesterol-positive lipids in interstitium and Sertoli cells, disintegration of sperm bundles, and spermatogenesis. (3) In the thumb pads a regression of the epidermis and in the seminal vesicles a reduction of the epithelium. The thyroids remained unaffected. Comparing these results with each other and with those of earlier experiments by the same authors, it was concluded that high temperatures diminished the ICSH activity of the pituitary and thus caused a reduction in the production and secretion of male sex hormone, and degenerative changes in the Sertoli cell-sperm bundle system. Moreover, high temperatures were believed to accelerate the FSH activity of the pituitary resulting in the disappearance of old remnants of sperm bundles and Sertoli cells, and subsequent spermatogenesis. The internal rhythm, regulating the sensitivity of the germinal epithelium to gonadotrophin, was thought to manifest itself by a varying demand for FSH and a seasonal refractoriness of the primary spermatogonia to this hormone. Each of these two factors makes it impossible to transform the discontinuous spermatogenetic cycle of Rana temporaria into a continuous cycle.