There is an increasing need for age-dependent dosimetric models, and it would be desirable to develop these models in such a way that the uniformity and basic features of the standard man models are retained. Unfortunately, available data concerning the age-dependent retention of nuclides would rarely permit the identification of compartments, uptake fractions, and clearance times using the empirical fitting methods that characterize the development of many adult models. However, in cases where compartments can be made to correspond to physically identifiable processes or subsections within an organ, it may be possible to combine relatively extensive information concerning the development of the human body with generally sparse nuclide-specific information to construct age-dependent compartmental models. In some cases there may be sufficient data to identify trends with age within compartments using empirical fitting techniques, provided the compartments have already been identified on physical bases. To obtain models that describe changes in a continuous manner from birth through adulthood, it may be necessary in many cases to modify existing adult models to consider fewer (or more easily identifiable) compartments. In this article we describe an age-dependent model for retention of ingested 90Sr in bone that exemplifies these concepts.