The aim of this study was to evaluate the effects of rat atrial-peptide type II (rAP-II) on testicular steroidogenesis by isolated adult rat Leydig cells. rAP-II stimulates testosterone secretion. The maximal stimulatory effects of rAP-II on testosterone production occurred at the dose of 10(-11) M. At the same dose this peptide stimulates androstenedione and dehydroepiandrosterone-sulphate production, two important testosterone precursors of delta-4 and delta-5 steroidogenetic pathways, respectively. At higher doses, 10(-9) and 10(-7) M, the stimulatory effect of rAP-II on testosterone secretion is strongly declined, on androstenedione secretion is abolished, whereas on dehydroepiandrosterone-sulphate secretion remains unaffected. These data suggest that rAP-II at low doses exerts a stimulatory effect on the early steroidogenetic step by isolated adult rat Leydig cells. At higher doses this peptide seems to influence the testicular steroidogenesis, probably exerting a limiting reaction step involving the 3-beta-OH-steroid-dehydrogenase activity.