The effect of methylprednisolone on the distribution of myocardial flow during ischemia and after reperfusion was determined in 10 treated and 11 control dogs. Methylprednisolone, 30 mg/kg, was administered iv after occlusion. Transmural blood flow and the endo/epicardial ratio in the normal and ischemic zones was measured by injecting 141Ce- and 85Sr-labelled microspheres after 90 min of ischemia and 30 min after reperfusion, respectively. During ischemia, transmural blood flow (ml g-1 min-1) in the ischemic zone was higher in treated animals than in controls (0.25 +/- 0.04 vs 0.15 +/- 0.03; P less than 0.05) but not different in the normal zone for both groups (1.36 +/- 0.14 vs 1.15 +/- 0.10; P greater than 0.05). The endo/epicardial ratio of the ischemic zone was low but similar for both groups during ischemia (0.43 +/- 0.07 vs 0.40 +/- 0.09; P greater than 0.05). After reperfusion, transmural blood flow increased significantly in treated (0.25 +/- 0.13 vs 1.36 +/- 0.13; P less than 0.0005) as well as in control animals (0.15 +/- 0.03 vs 1.63 +/- 0.14; P greater than 0.0005), reaching similar values (1.36 +/- 0.13 vs 1.63 +/- 0.14; P greater than 0.05). The endo/epicardial ratio also increased during reperfusion in both treated (0.43 +/- 0.07 vs 1.12 +/- 0.12; P less than 0.0005) and control animals (0.40 +/- 0.90 vs 1.03 +/- 0.19; P less than 0.025). These data show that although methylprednisolone-treated dogs had higher myocardial blood flow during ischemia, after reperfusion the increase in flow in the ischemic zone of treated and control dogs was of the same magnitude.