Rats were treated by exposure of the scrotum to a temperature of 43 degrees C for 30 min or bilateral ligation of the vasa efferentia and bled at 0, 3, 7, 14 and 21 days after treatment. In heat-treated rats FSH levels rose linearly from pretreatment levels while those in efferenticectomized animals remained unchanged for 3 days before increasing. In both groups FSH concentrations reached similar maximum values after 7 days and were significantly higher than those of intact controls at 7, 14 and 21 days. LH levels, although not generally different from those in the controls, rose from pretreatment levels in parallel with FSH. No differences were found in testosterone concentrations in any of the groups. Histological examination at 3 weeks after treatment confirmed that the germinal epithelium consisted mainly of spermatogonia and Sertoli cells. The cytoolgical appearance and lipid content of the Leydig cells of the aspermatogenic testes were indistinguishable from those of the controls and the weight and histological appearance of the accessory sex organs and the fructose content of the coagulating glands were also normal. It is concluded that the sterilizing effects of heat treatment and efferentiectomy are independent of changes in Leydig cell function and that the increase in gonadotrophin levels is related to the germ cell degeneration.