The aim of this study was to examine the possibility that atrial natriuretic peptide-stimulated testosterone production by mouse Leydig cells results from an activation of cAMP-dependent protein kinase (kinase A) by cGMP. In these cells, both 8Br-cGMP and 8Br-cAMP could stimulate testosterone production, though the latter was approximately 50-fold more potent. Following the stimulation of the cells with the atrial peptide, a dose-related decrease in the cellular protein-bound cAMP accompanied by a concomitant increase in the protein-bound cGMP was observed. The steroidogenesis stimulated by both human chorionic gonadotrophin (hCG) and atrial peptide was inhibited in a dose-dependent manner by a cAMP antagonist, adenosine 3',5'-cyclic monophosphothioate, Rp-isomer (RpcAMPS). In a cell-free [3H]cAMP binding assay, we have shown that unlabelled cGMP and RpcAMPS could competitively inhibit the [3H]cAMP binding, confirming that cAMP, RpcAMPS and cGMP could bind to the same binding protein. Finally, in a cell-free kinase A assay system, we have demonstrated that in lysates prepared from either atrial peptide or hCG-stimulated cells, the cellular kinase A was activated to an equal extent. We conclude from the data obtained that cGMP can bind to the cAMP-binding sites of kinase A and thereby brings about a promiscuous activation of this kinase. This appears to be an underlying mechanism by which atrial peptide hormone is able to stimulate the steroidogenesis in mouse Leydig cells.