Primary pituitary cell cultures from sexually mature adult male African catfish, Clarias gariepinus, were used to study the regulation of LH biosynthesis by sex steroids. The cell cultures were exposed to testosterone (T), estradiol (E₂), or 5α-dihydrotestosterone (DHT), a nonaromatizable analogue of T, and to the likewise nonaromatizable 11-ketotestosterone (KT) and 11β-hydroxyandrostenedione (OHA), physiologically relevant androgens in fish. Both T and E₂ elevated glycoprotein α (GPα) and LHβ steady-state mRNA levels (quantified by RNase protection assay), de novo synthesis (metabolic incorporation of radioactive amino acids and subsequent immune precipitation of LH), and release of preferentially newly synthesized LH, while DHT had no effect. Inhibiting the aromatase activity abolished the stimulatory effects of T. The effects of E₂ on LH mRNA levels and de novo synthesis were dose dependent. Incubation with 10 ng/ml KT elevated GPα and LHβ mRNA levels, while other concentrations of KT or all concentrations of OHA tested had no effect. The amount of newly synthesized LH, on the other hand, was decreased dose-dependently by OHA but not by KT. Since this OHA-induced decrease did not change the specific activity (dpm immune precipitable [³H]-LH/ng immune-reactive LH) of LH, we hypothesize that OHA exerted its effect by activating a crinophagic breakdown of secretory granules in catfish gonadotrophs. Electron microscopic examination of gonadotrophs after in vitro exposure to 50 ng OHA/ml revealed that breakdown organelles had increased in size significantly. We conclude that the balanced production of aromatizable (mainly stimulatory) and 11-oxygenated androgens (mainly inhibitory) may be an important factor in regulating the amounts of LH available for secretion in male African catfish.