The effects of prostacyclin (PGI₂ ) were compared with those of prostaglandin E₁ (PGE₁) on the cardiac hemodynamics (coronary blood flow, systemic blood pressure, cardiac output and max dp/dt of the left ventricle) in closed-chest dogs. Moreover, the effects of PGI₂ were compared with those of PGE₁ on the concentrations of the coronary arterial and myocardial cyclic adenosine 3': 5'-monophosphate (cyclic AMP) and cyclic guanosin 3': 5'-monophosphate (cyclic GMP) in open-chest dogs. The intraventricular and intravenous injection of 0.1 to 4.0 μg/kg of PGI₂ increased the coronary blood flow and decreased the mean systemic blood pressure. Comparison of dose response curves for PGI₂ and PGE₁ in relation to the coronary blood flow indicated that PGI₂ was 4 times as potent as PGE₁ . On the other hand, the effect of PGI₂ on the systemic blood pressure was twice as potent as PGE₁ . The concentration of the coronary arterial cyclic AMP was significantly increased by the administration of PGI₂ and PGE₁ (control group: PGI₂ group was 0.201±0.022 vs. 0.264±0.017 pmoles/mg tissue, p<0.05; control: PGE₁ was 0.201±0.022 vs. 0.286±0.027 pmoles/mg tissue, p<0.05). The concentrations of the coronary arterial cyclic GMP were not significantly changed by the administration of PGI₂ and PGE₁, nor were those of myocardial cyclic AMP and cyclic GMP. The changes in cyclic nucleotide levels and cardiac hemodynamics induced by PGI₂ were qualitatively similar to those induced by PGE₁, but the change caused by PGI₂ was greater than PGE₁ in the coronary and systemic hemodynamics.