The effects of testosterone propionate (1 mg/day) on the synthesis and circulating levels of FSH and LH were studied in normal adult male rats. The pituitary and serum gonadotrophins were measured by double antibody radioimmunoassay. The de novo synthesis of gonadotrophins was assessed by the rate of in vitro incorporation of [3H]leucine into the immunoprecipitable FSH and LH. After 4 days of treatment with testosterone propionate the circulating LH levels dropped significantly, while FSH remained unchanged. Pituitary LH content and concentration declined significantly after 1 day, and incorporation of [3H]leucine into the immunoprecipitable LH became undetectable 4 days after initiation of treatment. Pituitary FSH content and concentration showed a significant increase after the 4th day of treatment. A slight tendency towards increased incorporation of [3H]leucine into FSH was observed throughout the treatment period, although it was statistically not significant. The data provide direct evidence for a differential effect of TP on FSH and LH production by the pituitary and show that the decrease in the pituitary and plasma levels of LH in testosterone treated rats is due to the decrease in LH synthesis.