The dose-response curves of calcium concentration in the initial reperfusate to mechanical recovery of ischemic myocardium was investigated with an isolated working rat heart preparation. After 20 mins of normothermic ischemic arrest, the recovery of aortic flow by reperfusion with Krebs-Henseleit buffer was 54.0 +/- 2.4% (mean +/- S.E.M.). Reduction of calcium concentration during 10 mins of initial reperfusion after 20 mins of normothermic ischemic arrest increased or decreased the recovery of aortic flow with a bell-shaped dose-response curves obtained. The maximal percent recovery of aortic flow was 75.2 +/- 1.7 at 0.5 mM calcium with normal potassium-normal magnesium solution, 74.9 +/- 3.1 at 0.1 mM with high potassium (20 mM)-normal magnesium solution, 65.4 +/- 1.5 at 1.2 mM with high potassium-high magnesium (20 mM K, 16 mM Mg) solution, and 73.4 +/- 2.7 at 1.3 mM with normal potassium-high magnesium (16 mM) solution. All of them were significantly better than that of the control group (P less than 0.01). A high concentration of potassium and magnesium affected the dose-response of calcium in the initial reperfusate. These results suggest that the control of calcium concentration in the reperfusate is important during the early stages of reperfusion, and reperfusion injury may be considerably reduced.