1. The effect of intracellular perfusion with cyclic AMP and cyclic GMP on Ca2+ current (ICa) was studied in single cells isolated from frog ventricle using the whole-cell patch-clamp technique and a perfused pipette. 2. Intracellular perfusion with cyclic GMP (0.1-20 microM) had no effect on the basal ICa. However, when ICa was increased by isoprenaline or by intracellular perfusion with cyclic AMP, perfusion with cyclic GMP (20 microM) reduced ICa by an average of 67%. The effect of cyclic GMP on ICa elevated by cyclic AMP was reversible. A half-maximal effect of cyclic GMP was observed at 0.6 microM. Cyclic GMP had no significant effect on the shape of the ICa current-voltage relationship. 3. The effect of cyclic GMP was specific to the 3',5' form; 2',3'-cyclic GMP had no effect. 4. The effect of cyclic GMP was apparently not mediated by stimulation of cyclic-GMP-dependent protein kinase because 8-bromo-cyclic GMP, a very potent activator of the protein kinase, was without effect. 5. Cyclic GMP had no effect on ICa elevated by the non-hydrolysable 8-bromo-cyclic AMP. The effect of cyclic GMP on cyclic-AMP-elevated ICa was partially blocked by the phosphodiesterase inhibitor, methylisobutylxanthine. Thus, it was hypothesized that the effect of cyclic GMP was mediated by hydrolysis of cyclic AMP as a result of a stimulation of a cyclic nucleotide phosphodiesterase by cyclic GMP. 6. The dose-response curve for cyclic AMP on ICa was well fitted by the Michaelis equation with a K50 (i.e. concentration of cyclic AMP at which response is 50% of the maximum) of 0.7 microM and a maximal 11-fold stimulation of ICa. Cyclic GMP shifted the curve one log unit to the right and decreased the maximal stimulation to 8.6-fold. Thus, the effect of cyclic GMP appeared uncompetitive. 7. The products of cyclic AMP and cyclic GMP hydrolysis, 5'-AMP and 5'-GMP, had no effect on ICa. Furthermore, strong buffering of intracellular pH did not reduce the effect of cyclic GMP. 8. It is proposed that cyclic-GMP-stimulation of a cyclic nucleotide phosphodiesterase may be one of several mechanisms by which acetylcholine regulates ICa.