Abstract: Neriifolin, a crystalline glycoside, was isolated from the kernels of Thevetia peruviana by Tai Ke-sun (1962). The results of pharmacological studies on this glycoside are reported as follows: (1) The lethal dose of biological activity was found to be 0.1508 mg/kg ±0.0059(S.E.)for cats and 0.4955 mg/kg±0.0362 (S.E.) for guinea pigs. (2) In intraportal infusion experiments with cats, 27.12% of neriifolin was found to be eliminated in liver. (3) The enteral absorption of neriifolin after intraduodenal injection in cats was slow. 4, 6, and 8 hours after injection, the amount of absorption was found to be 31.2, 33.8 and 37.1% respectively. (4) By means of slow intravenous infusion, the elimination rate of neriifolin in cats was found to be 0.000878 mg/kg/hr. The ratio between the lethal dose of biological activity and the minimum lethal dose was 1.8. (5) The cats were injected intravenously with 50% cat units of neriifolin. After 24 and 48 hours, the cumulative rates were 23.7% and 10.81% of the intravenous dosage respectively. (6) When a single dose of neriifolin was injected to unanesthetized cats intravenously, the minimal lethal dose was 1.0—1.1 cat units, the minimal toxic dose was 0.6 cat units, and the maximal tolerance dose was 0.5 cat units. Therefore, the therapeutic index was 6.0.