1. The MLD50/50 for Sr90 in young Long-Evans rats was estimated to be 2.5 to 3.0 microcuries per gram of body weight. In rats which survived for fifty days, the histological changes in the tibia consisted in disruption of the epiphyseal line, osteosclerosis, aseptic necrosis of the trabecular bone, and depression of the myeloid elements.
2. The metabolism of Sr90 following intraperitoneal injection of an MLD50/50 dose was found similar to that for Sr89, Sr90,and Ca45 in tracer doses as reported by other authors. Sr 90 was quickly absorbed from the peritomseal cavity into the blood stream, and, at first, was distributed in part to the soft tissues; by the end of one to three hours, however, 75 to 80 per cent of the injected dose had been deposited in the skeleton. Up to 15 per cent of the injected isotope was excreted in the urine and up to 8 per cent in the faeces during the first twenty-four hours, urinary excretion being more variable than the fecal. Mobilization of the isotope from the skeleton was not appreciable during the five-day experimental period.