Adenosine (Ado) is a potent coronary vasodilator. Recent studies suggest that Ado may also have an important effect on myocardial carbohydrate metabolism. To determine whether Ado has a direct effect on myocardial glucose uptake, a recirculating, constant-flow, isolated rat-heart preparation was used. The hearts were perfused with Krebs-Henseleit buffer solution with an initial glucose concentration of 11 mmol/L. A control group was compared with hearts treated with Ado infusions (50 and 100 micrograms/min) or insulin (100, 200, and 300 microU/ml). In a separate series of experiments, nitroprusside was used to evaluate the effect of a nonspecific coronary vasodilator. The rate of glucose uptake was calculated as the amount of glucose removed from the perfusate normalized for heart weight and time. Developed pressure (DP) was assessed with an intraventricular balloon, and the coronary perfusion pressure (CPP) was monitored. At the end of 1 hour, the hearts were freeze-clamped and adenine nucleotide content was measured with HPLC. Ado treatment increased glucose uptake by 80% and 140%, respectively, at the two infusion rates (p less than 0.001). Myocardial adenosine triphosphate content was 18% and 26% higher in the Ado-treated hearts than in the controls (p less than 0.001). Ado also decreased the mean DP by 30% and 36% (p less than 0.001) and decreased CPP by 10% and 22%, respectively (p less than 0.001). Insulin increased glucose uptake in a similar dose response fashion but had no effect on myocardial nucleotide content, DP, or CPP. Nitroprusside decreased CPP but had no effect on glucose uptake, adenine nucleotide content, or DP. These results suggest that Ado may have a direct effect on glucose uptake independent of its properties as a coronary vasodilator.