A method is described by which the testes and left kidney of the rat can be isolated, maintained, and selectively perfused in vitro to examine the effect of potential gonadal toxins. The integrity of the system was confirmed by demonstrating appropriate increases in steroid (testosterone and estradiol) production and secretion in response to stimulation with chorionic gonadotropin; maintenance of testicular ATP levels; and failure to observe an increase in perfusate concentration of transaminase and lactic dehydrogenase over 2 hr of perfusion. After such confirmation, the effect of two potential gonadal toxins, ethanol and acetaldehyde, at levels commonly found in the plasma of men who abuse alcohol, was evaluated using the system. Both of these substances significantly (p less than 0.01) reduced the testosterone production and secretory response of the isolated perfused testes. The advantages of such a system are that the effect of specific potential toxins independent of hepatic metabolism and/or hypothalamic-pituitary suppression by such substances can be identified and examined in a system free of extragonadal metabolism.