1. Crotonoside (Isoguanosine) has much more active than adenosine in reducing the blood pressure in rabbits and cats, in decreasing the tone of isolated intestinal strips of the rabbit, guinea pig and hamster and in stimulating the isolated uterus of guinea pigs and hamsters. 2. Crotonoside inhibits the growth of S-18 and Ehrlich solid tumor in mice at the optimal doses of 96 mg/kg/day x 12 and 48 mg/kg/day x 12, with 1-T/C values of 65% and 6%, respectively.