The actions of human follicle-stimulating hormone (hFSH) and its beta-subunit were examined in several assays in reptiles, including effects on lizard testicular activity (growth and androgen production) in vivo, and stimulation of androgen production by snake testes and competition for binding of 125I-labeled hFSH in lizards and snakes in vitro. Binding was also examined with mammalian tissues. The hFSH was highly steroidogenic in the snake and lizard; otherwise results were similar to those observed in mammals. In all cases, the potency of the beta-subunit was only a few per cent of the intact hormone. The potency of hFSH in vivo compared with NIH-FSH ovine standards was several 100 times greater than in vitro. Results for stimulation of androgen production in vivo closely paralleled those for binding assays in both reptiles and mammals. In contrast to previous results for ovine FSH beta-subunit, human FSH beta-subunit has little if any FSH biological activity in reptiles.