Mice: Parenteral administration (s.c., i.p.) of Simtrazene (centrazene) produced optimum survival effects at daily doses ranging from 5.6 mg/kg (72j mammary adenocarcinoma) to 16.8 mg/kg (spontaneous mammary tumors). In diet experiments optimum survivals were obtained only when tumor-bearing mice consumed much higher daily doses of centrazene (about 2000 mg/kg body weight). This suggested that the compound was poorly absorbed orally or else inactivated in the digestive tract. The weight gain and mortality of normal C3H mice were unaffected by subcutaneous doses of centrazene as high as 2000 mg/kg given daily for 10 days. In contrast, rats given subcutaneous doses of this compound ranging from 2000 to 0.4 mg/kg daily for 10 days showed various degrees of mortality and growth inhibition.