The myocardial protective effect of two cardioplegic solutions was studied after an ischemic period of 2 h in eight dogs. Group I received a high potassium solution (St. Thomas Hospital) and group II a sodium withdrawal solution with high colloid osmotic pressure (Eppendorf solution). With the exception of a prolonged myocardial K+ washout (10 min) in group I, which was presumably responsible for fibrillation in the early reperfusion period, no major metabolic or perfusion differences between the two groups were observed. After 30 min of reperfusion, postischemic LV function (-43%) and O2 uptake (-43%) were equally reduced in both groups.