Abstract: Neriifolin is a cardiac glycoside isolated from the kernels of Thevetia neriifolia Juss. Diacetylneriifolin was obtained by acetylation of neriifolin. The minimum lethal doses of neriifolin and diacetylneriifolin in pigeons were found to be 0.217±0.008 mg (s.e.)/kg and 0.649±0.024 mg/kg respectively. Therefore, the biological potency of neriifolin was reduced by diacetylation. Furthermore the onset of action of diacetylneriifolin became slower than neriifolin; and it possessed a greater cumulative action than neriifolin. In experiments in pigeons, diacetylneriifolin showed sedative effect in small doses, and induced hypnosis or anaesthesia in large doses, but neriifolin did not give any such effect. Diacetylneriifolin prolonged the hexobarbital-sleeping time and antagonised the excitatory action of caffeine in mice receving intraperitoneal does of 1.0mg/kg.Hence it was a depressant of central nervous system.