The action of aconite on the sodium plus potassium activated ATPase activity in the rabbit red cell membrane has been investigated and the experiments were also designed to determine the mechanism of action of aconite on the ATPase activity. The following results were observed. 1. The activity of the NaK ATPase from red cell membrane is stimulated by aconite, and the concentration of aconite for maximal activity is about 80 mg%. The pH optimum for the aconite sensitive component is 8.0. 2. The activating effect of aconite on the ATPase, with a given concentration of sodium in the medium, is increased by raising the potassium concentration but activity ratio is decreased. 3. The activating effect of aconite on the ATPase, with a given concentration of potassium in the medium, is increased by raising the sodium concentration but activity ratio is decreased. 4. The action of aconite on the ATPase activity is inhibited by calcium ions and the effect of inhibition is increased by small amounts of calcium but decreased by larger amounts. 5. The activating effect of aconite on the ATPase was not related to the sulfhydryl group of cysteine, the amino group of lysine, the hydroxyl group of threonine or the imidazole group of histidine. 6. The action of aconite on the ATPase activity is due to carboxyl group of the enzyme of NaK ATPase.