The effect of synthetic human atrial 28-amino acid peptide (α-human atrial natriuretic polypeptide, α-hANP) on coronary circulation and cardiac functions were examined in openchest dogs. Continuous infusion of α-hANP into the left anterior descending coronary artery (LAD) increased blood flow of the LAD in a dosedependent manner. Linear regression analysis revealed the relationship as logit (changes in mean coronary conductance (MCC)=1.45×log (coronary plasma concentration of α-hANP) + 7.51, (r = 0.87, n = 29). REC50 of α-hANP was 5.1×10(-6)M, where REC50 was the concentration to increase MCC to a half maximum MCC during reactive hyperemia after a 30-second coronary occlusion. A force straingauge arch was inserteed directly onto the myocardium supplied by the LAD. Indices of myocardial contractility measured, myocardial force (F), max dF/dt and LVmax dp/dt, were not altered by either bolus intravenous injection or continuous intracoronary infusion of α-hANP. These results suggest that α-hANP increased coronary conductance, but did not change myocardial contractility.