The effect of calcium hydroxide on the maximal seizure threshold was studied in 18-day-old rats using the up-and-down method. The maximal seizure was induced by administering an electric shock through the eyes. When calcium hydroxide was given orally once a day (0.04 m moles/kg) for 10 days from the 8th to 18th day after birth, the maximal seizure threshold was raised by 4.8 mA, which corresponded to 16% of the threshold current in the control. the serum calcium concentration was not significantly altered after the treatment. When calcium chloride was given intraperitoneally, the maximal seizure threshold markedly increased with the increase in serum calcium. It is suggested that the mechanism of calcium hydroxide is different from that induced by increasing the serum calcium.