Peripheral concentrations of FSH in the male rat seem to be regulated in part by a protein hormone, inhibin, which originates from the testes. In an attempt to ascertain which type of testicular cell secretes inhibin, groups of male rats were irradiated prenatally or on days 4, 6 or 8 of postnatal life, and killed at 21, 51 or 81 days of age together with castrated and intact controls. The concentrations of FSH and LH in the pituitary gland, and FSH, LH and testosterone in the plasma were estimated for each animal, and the numbers of each class of intratubular cell in the testes were calculated. Rats irradiated neonatally had fewer Sertoli cells than controls at all ages studied, while the numbers of Sertoli cells in rats irradiated prenatally were higher than those in controls on day 21. The number of spermatogenic cells was usually decreased in rats irradiated postnatally. In the rats irradiated prenatally normal numbers of spermatogenic cells were found at day 51. Numbers of spermatogenic cells were significantly correlated with the number of Sertoli cells at the ages of 51 and 81 days. The concentrations of FSH in the plasma usually increased in the postnatally irradiated animals on days 21 and 51, but not on day 81; prenatal irradiation did not result in altered levels of FSH at any age. Peripheral levels of LH and testosterone were not affected by irradiation. The concentration of FSH in the plasma was negatively correlated with the number of Sertoli cells in all age groups, whereas significant correlations between the level of FSH and the number of spermatogenic cells were only found at days 51 and 81. It is concluded from these data that the Sertoli cell is the most likely source of inhibin.