This article reports the dynamic changes of serum potassium concentration after cardiopulmonary resuscitation in patients with cardiac arrest. The mechanism and significance for hypokalemia after resuscitation are discussed.The serum electrolyte were measured after return of spontaneous circulation(ROSC) in 22 patients with cardiac arrest not due to kidney, pulmonary or metabolic disease. The result showed that as early as within three hours after ROSC hypokalemia were found in 12 cases (54.4%). The mean value of PHa was 7.266±0.045 in patients with hypokalemia and 7.294±0.048 in patients with normalkalemia, no alkalosis was occured. The concentration of serum potassium wewe not correlated with the doses of epinephrine nor the time needed for ROSC (r was 0.139 and 0.168 respectively). It indicated that hypokalemia after CPR is not caused by alkalosis or exogenous epinephrine. It is proposed that this may be due to K transfered into cells from extracellular fluid through increasing endogenous catecholamine release. The serum potassium in 12 cases with hypokalemia returned to normal quickly within 24 hours after ROSC. The difference between CPCR success rate in both groups were not significant satistically (58.3% vs 50% P>0.05). It is suggested that hypokalemia after CPR is not important prognostic factor in patients with cardiac arrest.