Proposed radiopharmaceutical schedules based on readily measured parameters (age, height, weight, surface area) have been considered with respect to their applicability in nuclear medicine. Although schedules based on age are considered to be inappropriate, there are valid cases for schedules based on height, weight and surface area. For many radiopharmaceutical studies, the most appropriate schedules are based on weight or surface area. Examination of simple theoretical physical models, including the influence of attenuation, suggests that the optimum administered amounts of radiopharmaceuticals fall mainly within the region bounded by these two schedules. The results of recent clinical studies designed to test the validity of different schedules are summarized. In general, they support the predictions of theoretical models, but also show how simple models can be influenced significantly by the age dependency of radiopharmaceutical biodistribution. The schedule based on surface area (or height) is less likely than that based on weight to require the identification of minimum administered amounts of radiopharmaceutical to preserve image quality in small children. However, recent studies have shown that the most appropriate schedule for regional cerebral blood flow with 99Tcm-HMPAO is that based on weight, without the need for a minimum activity. Paediatric radiation dosimetry is briefly summarized to indicate some recent innovations in methodology. Effective doses per unit of administered radiopharmaceutical (mSv MBq-1) have been calculated using five paediatric phantoms for a number of radiopharmaceuticals commonly used in children. Values of total effective dose resulting from the application of the weight and surface area schedules are presented, based on the adult reference amounts of administered radiopharmaceutical proposed by the Paediatric Task Group of the European Association of Nuclear Medicine. Although some values of effective dose exceed 10 mSv for the surface area schedule, the majority of values are less than 5 mSv.