1. The effects of high extracellular potassium on hypoxia-induced atrial activity and metabolic charge were studied in isolated rat atria. 2. After hypoxia (30 min), contractile tension strongly decreased and diastolic tension increased, while frequency did not change. Adenine nucleotides and creatine phosphate levels did not change, although a significant increase in lactic acid content was observed. 3. High [K+] mostly countered the hypoxia-induced increase in diastolic tension. Moreover, in the presence of high [K+], the hypoxia-induced increase in lactic acid was not significantly different from normoxic controls. 4. Glibenclamide (0.1 microM), a selective K+ATP channel blocker, did not improve the hypoxia-induced depression of atrial function. 5. The physiopathological role of extracellular potassium during cardiac hypoxia is discussed.