Characterization of pituitary LH and FSH from intact and castrated male monkeys showed that gonadotropins from intact males are smaller in average apparent molecular size, and disappear from the circulation of test rats more rapidly than hormones from castrated males. Furthermore, intact male monkey LH and FSH are smaller and are cleared from the rat circulation faster than the gonadotropins from intact female monkeys. We conclude that 1) in the rhesus monkey changes in the properties of pituitary LH and FSH resulting from orchidectomy are similar to those which follow ovariectomy, 2) the response to gonadectomy of the male monkey with regard to the molecular properties of FSH is opposite to that of the male rat, and 3) the relationship between the influence of male and female sex hormones on the apparent molecular size of FSH in the monkey is the reverse of that in the rat.