In an effort motivated largely by the Chernobyl nuclear accident, the International Commission on Radiological Protection (ICRP) is developing age-specific biokinetic models and dose coefficients for environmentally important radionuclides. This paper describes the ICRP's age-specific biokinetic model for uranium. The model is constructed within a physiologically based framework originally developed for application to the alkaline earth elements but sufficiently general to apply to the larger class of bone-volume-seeking elements. Transfer rates for a reference adult are based mainly on: 1. measurements of uranium in blood and excreta of several human subjects who were intravenously injected with uranium; 2. postmortem measurements of uranium in tissues of some of those subjects; 3. postmortem measurements of uranium in tissues of occupationally and non-occupationally exposed subjects; 4. data on baboons, dogs, and smaller laboratory animals exposed to uranium for experimental purposes; and 5. consideration of the physiological processes thought to control retention and translocation of uranium in the body. Transfer rates for the adult are extended to children by application of a set of generic assumptions applied by the ICRP to calcium-like elements. These assumptions were derived mainly from observations of the age-specific biokinetics of the alkaline earth elements and lead in humans and laboratory animals but are consistent with available age-specific biokinetic data on uranium.