We studied serum electrolyte levels, blood gas values, and diuretic history in 115 patients immediately after resuscitation from out-of-hospital ventricular fibrillation. Comparisons were made with 84 patients admitted for acute myocardial infarction and with 88 ambulatory patients with coronary heart disease. Admission serum potassium values of 3.60 mEq/L or lower were found in 56 (49%) of the resuscitated patients, compared with 16 (19%) and eight (9%) of the comparison groups, respectively. In the resuscitated patients, the mean serum potassium level was 3.70±0.72 (SD) mEq/L, compared with 4.09 ±0.66 and 4.17 ±0.35 mEq/L in the respective comparison groups. Reduced serum potassium concentration was not related to simple extracellular alkalosis. However, the absence of a causative relationship to diuretic use suggested that it occurred during cardiac arrest and resuscitation. Rapid correction of extracellular acidosis by bicarbonate therapy and ventilation may have produced a shift of extracellular potassium into the intracellular compartment even in the absence of extracellular alkalosis.