The retention of intramuscularly injected monomeric 239Pu has been studied in the tree shrew (Tupaia belangeri), a primitive prosimian species. Data for the first 2 yr are presented. Resorption from the injection site is rapid, and the main depository organs are skeleton and liver with approximately 50 and 20% of the absorbed dose, respectively. The half life of retention in skeleton is approximately 500-600 days and in liver approximately 150 days, these half lives are compared to data for the retention of transuranium elements in other animal species. Indications of liver lesions were found in 5 animals and 1 animal died at day 790 with an osteosarcoma.