Abstract: The cardiotonic effects of Strophanthus divaricatus (Lour.). Hook et Arn. had been confirmed by Lue Fu Hua and Kiang Ming Sing and its therapeutic effects on the cardiac failure was also obtained clinically by other authors. But there are no comparative stu- dies upon the therapeutic index and relative potency between Strophanthus divaricatus (Lour.) Hook et Arn. (I) and g-Strophantin (II) on the same experimental bases. Experimental results indicated that the LD₅₀ (i.v.) of I and II was 6.93 (5.45- 7.68) mg/kg and 3.68 (3.09-4.09) mg/kg respectively in mice; 0.430 (0.412-0.442) mg/kg and 0.172 (0.163-0.180) mg/kg respectively in pigeons; and the minimal lethal dose of I and II was 0.3375±0.0125 mg/kg and 0.1575±0.0056 mg/kg respectively in cats. The ED₅₀ (i.v.) of I and II was 1.06 (0.92-1.23) mg/kg and 0.72 (0.62-1.23) mg/ kg respectively in mice (percentage of falling according to Koch-Kneip's Method) and it was 0.164 (0.147-0.183) mg/kg and 0.050 (0.0443-0.0559) mg/kg respective- ly in pigeons (vomiting). The therapeutic index of I and II was 6.54 (5.15-8.31) and 5.11 (4.12-6.34) respectively in mice and it was 2.62 (2.34-2.93) and 3.44 (3.19-3.93) respectively in pigeons. The difference of therapeutic index between I and II was not significant in mice, but was significant in pigeons, where the therapeutic index of II was somewhat larger than that of I. But from the clinical point of view although I has a smaller therapeutic index, its toxicity is also lower than that of II, and in the clinical use, it may have a greater therapeutic width than II. The doses may be controlled more easily than II. This illus- trate an advantage of I over II. The cardiac rates of pigeons was decreased significantly to about the same degree following a dose of 1/3 LD₅₀ of either I or II, but the duration of the effects of I was somewhat shorter than that of II. Following a dose of 1/2 LD₅₀ the decrease in the cardiac rates was to a greater extend, where the duration of I was somewhat longer than II. Following a dose of 2/3 LD₅₀ the cardiac rates was however significantly accelerated by either I or II, without difference between them. The detoxication rate of I was much slower than II in pigeons. It may be concluded, that, if the same effects were desired, the dose of I should be increased as large as two times that of II.