Reperfusion of ischemic myocardium is associated with phospholipid degradation and corresponding changes in membrane fluidity. Dexamethasone (1.25 mg/kg i.v.) was evaluated in the pig, with pretreatment of the animal 1 1/2 hours before ischemic insult. The isolated perfused in vivo pig heart model was subjected to 60 minutes of regional ischemia of the left anterior descending coronary artery. The ischemic heart was then subjected to 60 minutes of global hypothermic cardioplegic arrest followed by 60 minutes of reperfusion, including reperfusion of the ischemic left anterior descending coronary artery region. Phospholipase A2, arachidonic acid, total free fatty acids, myocardial microviscosity, coronary blood flow, myocardial oxygen consumption, creatine kinase release, and regional and global myocardial function were measured. Dexamethasone pretreatment resulted in dramatic inhibition of phospholipase A2 activity accompanied by a reduction in arachidonic acid and total free fatty acid levels. Myocardial microviscosity (the inverse of membrane fluidity) was significantly increased only in untreated animals. Coronary blood flow and myocardial oxygen consumption were maintained at preischemic levels only in the dexamethasone-treated animals and were significantly reduced in the control group. Creatine kinase release increased nearly six times in control animals only while remaining stable in the dexamethasone-treated group, and regional and global myocardial contractility and compliance were improved dramatically in the dexamethasone-treated animals. These results indicate that dexamethasone enhances myocardial function by preserving membrane structure through inhibition of phospholipase activation, thereby preventing phospholipid degradation and maintaining membrane integrity and fluidity.