The authors examined whether partial reperfusion protects against reperfusion ventricular fibrillation (VF) following severe acute myocardial ischemia. Fifty-seven dogs were divided into two groups. In group A (n = 21), the left anterior descending coronary artery was occluded for 10 minutes, followed by full reperfusion. In the remaining 36 dogs (group B), myocardial ischemia was induced by retrograde blood flow (RBF) for 10 minutes. Thereafter, these dogs were divided into three subgroups: in group B1 (n = 10), full reperfusion was made by a carotid-left anterior descending coronary artery bypass; in group B2 (n = 13), partial reperfusion was achieved by collateral flow into the ischemic zone due to stopping RBF; in group B3 (n = 13), RBF was continued for an additional 5 minutes. During 10 minute ischemia, the myocardial blood flow at the ischemic zone measured by the H2 gas-clearance method was 12.3 +/- 2.0 ml/min/100 g in A, 3.4 +/- 0.9 ml/min/100 g in B1, 4.7 +/- 0.6 ml/min/100 g in B2, and 4.7 +/- 0.6 ml/min/100 g in B3 (A vs B1, p less than 0.02; A vs B2 and B3, p less than 0.01). Maximal ST-segment elevation was 11.4 +/- 1.8 mV in A, 28.2 +/- 2.7 mV in B1, 25.1 +/- 3.0 mV in B2, and 27.0 +/- 1.9 mV in B3 (A vs B1, B2, and B3, p less than 0.001). Maximal conduction delay was 48.6 +/- 9.4 ms in A, 106.4 +/- 5.2 ms in B1, 101.6 +/- 9.9 ms in B2, and 91.2 +/- 5.1 ms in B3 (A vs B1, B2, and B3, p less than 0.001). The incidence of reperfusion VF was 14% (3/21) in A, 80% (8/10) in B1, and 69% (9/13) in B2 (A vs B1, p less than 0.001; A vs B2, p less than 0.005). In group B3, VF occurred in only 1 of 13 dogs for the additional 5 minutes. It is concluded that reperfusion VF occurred frequently when ischemia was severe even though the duration of ischemia was short (B1), and that reperfusion VF was not prevented by partial reperfusion when the ischemia was severe (B2).