Female specimens of Heteropneustes fossilis were exposed to a sublethal concentration of the organochlorine agricultural pesticide gamma-BHC (16 mg liter-1) for 4 weeks during the reproductively active preparatory and prespawning phases of the annual reproductive cycle. The effects of gamma-BHC on the gonadosomatic index (GSI) and on the histological changes in ovary, cholesterol-free cholesterol (CF), esterified cholesterol (CE), and the sex steroids testosterone-estradiol-17 beta, and 17 alpha-hydroxyprogesterone in plasma and ovary were monitored. Mystus gonadotropin (mGTH) was administered along with gamma-BHC exposure during the preparatory and prespawning phases. gamma-BHC decreases GSI during both phases; however, simultaneous administration of mGTH recovered the effect. gamma-BHC exposure caused inhibition of ovarian growth, resulting in a large number of stage I oocytes and few stage II oocytes, while at higher doses, mGTH reversed the impact of gamma-BHC, and a large number of stage III and vitellogenic oocytes were recorded. This pesticide also inhibited deesterification of CE to CF. Additionally, it impaired ovarian steroidogenesis and secretion of steroids into plasma. Lower doses of mGTH (0.1 and 1.0 microgram/fish) were ineffective in modulating the impact of gamma-BHC on lipids and steroids. However, higher doses (10 and 20 micrograms/fish) effectively revoked the impact of gamma-BHC, routed through the hypothalamo-hypophyseal-ovarian axis and directly upon the ovary.