Good evidence exists to indicate that the vasodilating effect of adenosine is mediated by cell surface receptors on vascular smooth muscle cells. The mechanism of transmembrane signal transduction for adenosine, however, is not fully understood. Since cGMP is a second messenger known to mediate vasodilation, I have examined the effect of adenosine on the intracellular concentration of cGMP in vascular smooth muscle cells from rat aorta. I found that adenosine at 10(-9) to 10(-5) M led to an increase in intracellular cGMP levels in a dose-dependent fashion. The effect of adenosine on cyclic guanosine inorganic monophosphate (cGMP) could be mimicked by the A-type receptor agonists N6-cyclohexyladenosine and 5'-N-ethylcarboxamidoadenosine and was attenuated by the A-receptor antagonist theophylline. The order of potency of the adenosine analogues was N6-cyclohexyladenosine greater than 5'-N-ethylcarboxamidoadenosine greater than adenosine. These findings suggest that the effect of adenosine on cGMPi is mediated by A1-type cell surface receptors. Concerning the mechanism by which adenosine could elevate cGMPi, I found that the effect of adenosine on cGMPi was potentiated by the cGMP phosphodiesterase-specific inhibitor M & B 22948. Moreover, I found that N6-cyclohexyladenosine, 5'-N-ethylcarboxamidoadenosine, and adenosine stimulated a guanylate cyclase in homogenates of the cultured smooth muscle cells in a dose-dependent fashion with the same order of potency as their effects on cGMPi. Further evidence was obtained to indicate that adenosine and its analogues stimulated a particulate guanylate cyclase activity, whereas they did not alter soluble guanylate cyclase activity. Since cGMP is known as a second messenger mediating relaxation of vascular smooth muscle cells, the results obtained in this study could suggest that adenosine exerts its vasorelaxing effect by activating an Ai-receptor-linked guanylate cyclase.