The effects of the administration of estradiol benzoate (EB) or testosterone propionate (TP) for 30 days were investigated in respect to the response of the hypothalamo-hypophysial-ovarian system of the catfish, Heteropneustes fossilis, during the prespawning and spawning periods. A low dose of EB (2.5 µg/day) was ineffective. Treatment with a higher dose of EB (5 µg/day) or TP (50 µg/day) brought about a depletion of neurosecretory material (NSM) in the perikarya of the nucleus lateralis tuberis (NLT) and the nucleus preopticus (NPO), a reduction in the size and number of basophils in the pituitary, and the inhibition of vitellogenesis in the ovaries. The NLT rather than the NPO was greatly influenced by EB or TP treatment. Furthermore, the effect of EB on the NLT-hypophysial-ovarian system was considerably more marked than the effect of TP. Cessation of treatment with EB or TP resulted in a restoration of the activity of the NLT, the hypophysis, and the ovaries. Evidence is presented to show that the inhibition exerted by EB or TP on the hypothalamus is tentative and that this inhibitory influence is gradually abolished following the withdrawal of EB or TP administration.