The action of theobromine on the sodium plus potassium activated ATPase activity In the rabbit red cell membrane has teen investigated and the experiments were also designed to determine the mechanism of action of theobromine on the ATPase activity. The following results were observed. 1. The activity of the NaK ATPase from red fell membrane is stimulated by theobromine, and the concentration of theobromine for maximal activity is about 3mM. 2. The activating effect of theobromine on the ATPase, with a given concentration of potassium in the medium, is increased by raising the sodium concentration but activity ratio is decreased. 3. The activating effect of theobromine on the ATPase, with a given concentration of sodium in the medium. is increased by the raising the potassium 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 of the enzyme by theobromine is increased by small amounts of calcium but decreased by larger amounts. 5. The activating effect of theobromine on the ATPase was not related to the hydroxyl group of threonine and imidazole group of histicline. 6. The activating effect of theobromine on the ATPase is due to sulfhydryl group, amino group and carboxyl group of the enzyme of NaK ATPase.