This study was designed to clarify the relationship between the change in H<sup>+</sup> and the nucleoside concentration at a constant coronary flow rate using a Langendorff system. Wistar rat hearts were perfused with Krebs-Henseleit solution (control, pH 7.4) modified to be either acidotic or alkalotic. Coronary perfusion presssure (CPP), left ventricular pressure, and myocardial oxygen consumption were decreased in the acidotic perfusate. On the other hand, these parameters increased in the alkalotic perfusate. Adenosine production in the acidotic perfusate significantly decreased while it significantly increased in the alkalotic perfusate. Furthermore, there was a significant positive correlation between both the percent change in adenosine production and the percent change in CPP in acidosis and alkalosis. It is suggested from these results that the hydrogen ion affects the adenosine metabolism and that the change in coronary blood flow by H<sup>+</sup> is at least partly regulated by adenosine.