The intracellular cation contents were determined in isolated perfused rat heart using cobaltic EDTA as a marker of the extracellular space. In hearts in which Na+ accumulation was induced with monensin, a Na+ ionophore, during 20 min-ischemia which otherwise did not result in accumulation of Na+, the levels of Na+ and Ca2+ were significantly higher after reperfusion with a significant decrease in K+. While the recovery of the cardiac mechanical function (CMF) was complete after reperfusion in control hearts, the recovery was incomplete in monensin-hearts. Dichlorobenzamil (DCB), the most specific inhibitor of Na(+)-Ca2+ exchanger, infused for 10 min before induction of ischemia in a dose of 10(-5) M, which produced a definite suppression of CMF (over 80%), inhibited the accumulation of Ca2+ and Na+ and the loss of K+ and ATP after 40 min-ischemia and reperfusion. The same dose of DCB given for 3 min before induction of ischemia and after reperfusion, which produced a less than 20% inhibition of CMF, failed to prevent the Ca2+ accumulation after 40 min-ischemia and reperfusion. These findings are at variance with the idea that the accumulation of Na+ during ischemia and the consequent augmented operation of Na(+)-Ca2+ exchange is responsible for accumulation of Ca2+ after reperfusion.