The macroscopic and microscopic distribution of intramuscularly injected, essentially monomeric 239Pu, was studied in the tree shrew (Tupaia belangeri). Data for the first 15 months after injection are presented. About 50 per cent of the absorbed dose is deposited in the skeleton. The microscopic distribution was analysed in femora, humeri, tibiae and lumbar vertebrae. The initial bone surface activity is about 11 Bq/cm2 per 37 kBq/kg injected dose (related to a bone uptake of 50 per cent) and is similar in all four bones. At 462 days after injection the surface activity in the tibiae and humeri is comparable with the initial concentration, whereas only 50 per cent and 37 per cent of the initial surface activity is present in the femora and lumbar vertebrae respectively. A comparison with results from rats and dogs shows that at 5 days after injection the normalized endosteal surface activity is similar but marked species differences are found at 365 days after injection.