Spermatogenesis in rats was interrupted by local X-irradiation, heat or ligation of the testicular efferent ducts. A significant and specific rise in the serum level of FSH occurred 5--8 days after ligation of the efferent ducts, reaching twice the value observed in sham-operated controls by 21 days after the operation. After the testes were heated to 43 degrees C for 30 min, the serum levels of both LH and FSH were raised within 3 days and remained so up to 50 days after treatment. After X-irradiation, no changes in the concentration of FSH were observed in the first 21 days after treatment, but the serum levels of both gonadotrophins were increased at 49 days. By comparing the relative increases in the concentrations of FSH and LH after germ cell damage with those occurring after castration, it was evident that testicular androgens could account for only part of the normal feedback control of FSH secretion; at least one third of the inhibition of FSH secretion appeared to be due to non-androgenic sources, presumably 'inhibin'.

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Summary. The composition of fluid from the seminiferous tubules and rete testis of the rat has been studied. Differences in ionic, steroid and protein concentrations in the two fluids suggest that two different fluids are secreted, but from the numbers of sperm and the concentrations of inositol, it now seems more likely that all the fluid is secreted in the seminiferous tubules but that its composition is modified as it passes through the tubuli recti or when it reaches the rete testis. The permeability of the rete to potassium is appreciably higher than that to sodium ; the latter is comparable to the permeability of the tubules to both ions. The potential difference from blood is similar at the two sites (approx. 5 mV lumen negative).Movements of fluid along the tubules have been observed using small oil droplets. Introduction.

Results are presented on the changes in serum FSH, LH and testosterone levels and in testis weight after efferent duct ligation, X‐irradiation or heating the testes, or injections of antiserum to testosterone. These results can only be explained by the existence of a feed‐back substance “inhibin”, which is produced by the seminiferous tubules and which regulates the secretion of FSH by the pituitary. Several bioassay methods have been developed for the detection of inhibin‐like activity and have been used to follow the purification of this activity, mainly from ram rete testis fluid proteins. The active material exhibits an apparent heterogeneity, with apparent molecular weights of 90 000, 20 000 and < 5000.