Pituitaries from male and female fish were incubated with 1, 5, and 10 micrograms/ml of oestradiol-17 beta, testosterone, 17 alpha OH-progesterone, and 17 alpha,20 beta-dihydroxy-4-pregnan-3-one for 18 hr. Ultrastructural differences between control and experimentally treated glands were used to define the direct effects of these steroids on the gonadotropic cells of the adenohypophysis. The effects of the steroids differed according to sex and reproductive state of the donor animal. Oestradiol and testosterone stimulated gonadotropin secretion by active gonadotrophs, but inhibited it in inactive cells. Both the progesterones generally inhibited gonadotropin secretion although 17 alpha,20 beta-dihydroxy-4-pregnan-3-one had no effect on active gonadotrophs. The four steroids investigated all show potential for direct control of gonadotropin secretion in Poecilia latipinna although factors affecting the balance of these actions, and their relative importance in vivo, remain to be elucidated.