The effects of s.c. administration of oil, testosterone, 5 alpha-dihydrotestosterone, 5 alpha-androstane-3 alpha, 17 beta-diol and oestradiol-17 beta on plasma concentrations of LH and FSH were determined in 5 orchidectomized dogs. The dosages for the androgens and oestradiol-17 beta were 500 and 50 micrograms/kg body weight, respectively. Testosterone and oestradiol-17 beta significantly reduced plasma gonadotrophin concentrations, although the onset and duration of their suppressive effects differed. Dihydrotestosterone and oil had no effect on either gonadotrophin. Administration of androstanediol had no effect on plasma concentrations of LH but did cause a temporary and significant reduction in FSH. It is concluded that testosterone and oestradiol-17 beta are major regulators of gonadotrophin secretion in the male dog, but the 5 alpha-reduction of testosterone seems to play only a minor role in this control.