The purpose of this study was to determine if significant relationships exist between plasma and aortic cyclic GMP (cGMP) levels and pharmacodynamic effect after the i.v. administration of the cGMP-selective phosphodiesterase inhibitor zaprinast to conscious, spontaneously hypertensive rats. Zaprinast dose-dependently increased plasma and aortic cGMP levels at 10, 18 and 30 mg/kg and decreased mean arterial blood pressure (MAP) at 18 and 30 mg/kg. The concentrations of cGMP in the plasma and in the aorta were significantly correlated (r = 0.765, P < 0.0001). The changes in MAP were significantly correlated to aortic (r = -0.750, P < 0.0001) and plasma (r = -0.762, P < 0.0001) cGMP levels. We conclude that plasma cGMP may be an index of cGMP-selective phosphodiesterase inhibition in vivo.