The synthetic atrial peptides, rat atrial natriuretic peptide, atriopeptin I and atriopeptin II, stimulated testosterone production by mouse Leydig cells in a time- and concentration-dependent manner. The maximum stimulation of the steroidogenesis in response to the peptides was 6-10-fold over the basal level, as compared with 20-24-fold stimulation obtained with saturating concentrations of hCG. The stimulation of steroidogenesis by the most potent peptide, atriopeptin II, was markedly enhanced in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, suggesting an involvement of cyclic nucleotides. However, neither basal nor hCG-stimulated levels of cAMP were altered by the peptide, though testosterone production in response to submaximal concentrations of hCG was increased in the presence of atriopeptin II. The nature of the second messenger involved and the mechanism of action of the atrial peptides may be elucidated by further research in progress.