Isoproterenol ISO), in a single dose of 7.5 mg/kg body weight, was found to produce alterations of cardiac metabolism in dogs. After 2 hours, high energy phosphate stores and glycogen were reduced, whereas the levels of lactate and pyruvate, the lactate/pyruvate ratio, and myofibrillar ATPase activity were elevated. Ca2+ accumulation by sarcoplasmic reticulum (SR) and mitochondria were increased (p less than 0.01). After 24 hours, a partial recovery in the parameters studied could be observed. Only myofibrillar ATPase activity and the Ca2+ uptake by SR and mitochondria were lowered. When K+, Mg2+-aspartate (K,Mg-ASP) was administered concurrently with ISO, myofibrillar ATPase and Ca2+ accumulation by SR did not differ from controls 2 hours after ISO application; also, the other parameters exhibited a tendency to improve (p less than 0.01), but did not reach control levels. At 24 hours after ISO application, we could observe a similar effect of K,Mg-ASP in the prevention of Ca2+ overload accompanying metabolic changes.