The effects of NaHCO3 on the electrocardiogram of rats were studied in the induced hyperkalemia. The subjects were divided into 4 groups: the group 1 was normal control and the data on this normal control had teen obtained from the following three groups before administration of KCl or NaHCO3, the group 2 (KCl) was administered 40 ml per kg body weight of the 10 per cent KCl solution, the group 3 (NaHCO_3) was administered 40 ml per kg body weight of the 10 per cent NaHCO3 solution, and the group 4 (NaHCO3) was received 10 per cent KCl, which was followed by administration of 10 per cent NaHCO3 at one and half hours later. In KCl, the heart rate was decreased rapidly, and then maintained its level, later rapid decreasing heart rate was followed by the cardiac stand still. The mean electrical axis of QRS complex became progressively deviated to the left. The amplitude of T wave was increased transiently but was not changed thereafter. There was prolongation of the P-Q interval and the Q-T interval at the beginning and then they were shortened. In NaHCO3, the heart rate was decreased rapidly at the beginning, later showed a tendency of recovery. The mean electrical axis of QRS was not changed initially, but later became deviated to the left. The amplitude of T wave was not changed. There was prolongation of the P-Q interval and the Q-T interval at the beginning and then they were shortened. In KCl+NaHCO3, there were a tendency of recovery of both the amplitude of the T wave and the electrical axis of the QRS complex after administration of NaHCO3 but the heart rate was not recovered. There was prolonged P-Q interval, but the Q-T interval was relatively unchanged.