The interaction between renin, nitric oxide (NO) and its second messenger cGMP is controversial. cAMP is the stimulatory second messenger for renin, but is degraded by phosphodiesterases (PDEs). We have previously reported that increasing endogenous cGMP in rats by inhibiting its breakdown by PDE-5 stimulated renin secretion rate (RSR). This could be reversed by selective inhibition of neuronal nitric oxide synthase (nNOS). PDE-3 metabolizes cAMP, but this can be inhibited by cGMP, suggesting that renal cGMP could stimulate RSR by diminishing PDE-3 degradation of cAMP. Rats were anesthetized with Inactin prior to determination of blood pressure (BP), renal blood flow (RBF) and sampling of renal venous and arterial blood to determine RSR. In 13 rats, basal BP was 104 ±2 mm Hg, RBF was 6.1 ml/min/gkw and RSR was 2.9 ±1.4 ng AngI/hr/min. Inhibiting PDE-5 with 20 mg/kg bw ip Zaprinast did not change hemodynamic parameters, but increased RSR 5-fold to 12.2 ±4.9 ng AngI/hr/min (p<0.05). Renal venous cAMP was increased by Zaprinast from 93.8 ± 27.9 to 149.2 ± 36.0 pM/min/gkw (p<0.05). When another 10 rats were treated with the PDE-3 inhibitor Milrinone (0.4 µg/min over 30 min, which did not affect hemodynamics), RSR was elevated to 10.4 ± 4.4 ng AngI/hr/min. Milrinone also increased renal venous cAMP from 212 ± 29 to 304 ± 29 pM/min/gkw (p<0.025). Administration of Zaprinast to rats pretreated with Milrinone (n=10) did not further increase in RSR (7.5 ± 3.3 ng AngI/hr/min). These results are consistent with endogenous renal cGMP inhibiting PDE-3, which diminishes renal metabolism of cAMP. The resulting increase in cAMP serves as an endogenous stimulus for renin secretion. This suggests a pathway by which NO can indirectly stimulate RSR through its second messenger cGMP.