The action of pilocarpine 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 pilocarpine on the ATPase activity. The following results were observed. 1. The activity of the NaK ATPase from red cell membrane is stimulated by pilocarpine, and the concentration of pilocarpine for maximal activity is about 3 mM. The pH optimum for the pilocarpine sensitive component is 8.0. 2. The activating effect of pilocarpine 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 pilocarpine 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 NaK ATPase activity is increased by small amounts of calcium but decreased by larger amounts. The activity ratio of the enzyme by pilocarpine is decreased by small amounts .of calcium but decreased by larger amounts. 5. The activating effect of pilocarpine on the ATPase was not related to the sulfhydryl group of cysteine, the hydroxyl group of threonine or the imidazole group of histidine. 6. The activating effect of pilocarpine on the ATPase is due to amino group and carboxyl group of the enzyme of NaK ATPase