In the present study, dogs were pretreated with intravenous digoxin, 0.0125 mg/kg/day, for 6 to 7 consecutive days to achieve clinically relevant serum concentrations; untreated animals were used as control subjects. After pretreatment, nine digoxin-pretreated dogs and nine control dogs were anesthetized and subjected to a 60-minute occlusion of the left circumflex coronary artery, followed by 6 hours of reperfusion. Anatomic myocardial infarct size, expressed as a percentage of the areas at risk of infarction and as a percentage of the total left ventricle were: 20.2 +/- 3.3% control vs 35.4 +/- 6.2% digoxin-pretreated (p less than 0.05) and 8.6 +/- 1.3% control vs 14.7 +/- 2.5% digoxin-pretreated (p less than 0.05), respectively (2.04 +/- 0.37 ng/ml serum digoxin). Regional myocardial blood flow in the nonischemic and ischemic zones tended to be lower in digoxin-pretreated than in control animals at baseline testing and were significantly reduced in the anterior subendocardial sites of digoxin-pretreated dogs during ischemia and reperfusion. These data suggest that an exacerbation or enhancement of myocardial ischemia-reperfusion injury may occur in the presence of clinically observable serum digoxin concentrations.