The effect of FSH on the liberation of testosterone induced by LH in the Leydig cell has been investigated. Leydig cells were obtained from adult male rats, and were incubated with LH in one group and with LH and FSH in another group. Incubation was kept at 37 degrees C, 120 cycles/min, in oxygen-carbon dioxide (95:5) atmosphere. Liberated testosterone was measured by RIA, at different periods of incubation. Other group of cells, in identical experimental conditions, were incubated for 120 min with LH and FSH, where the FSH had been previously incubated with anti-FSH antibody. The cells incubated with LH and FSH liberated more testosterone than those incubated with LH. However this effect disappeared when the added FSH had been previously incubated with its antibody. The results show clearly that FSH liberates testosterone by the action of LH, which must be specific since it disappears when the hormone is incubated with its antibody.