Growth hormone (GH) secretion is controlled by many factors, including stage of development, age, gonadal steroids, body composition, nutritional state, time of day and whether the subject is asleep or awake. Understanding regulation of GH secretion is important since this hormone regulates not only growth, but also the partitioning of nutrients and body composition. There is increasing evidence that there is a basic ultradian rhythm of GH secretion. The NSF Center studies will be facilitated by 3 major efforts: (a) improvement of sensitivity of GH assays to permit accurate description of GH pulses; (b) use of biomathematical models to objectively determine GH pulse characteristics, as well as calculation of secretion rates to facilitate the study of the relationship between neural controls and GH secretion; and (c) use of the tau mutant hamster and the new mouse mutant animal models. By manipulation of the endogenous circadian clock in these animal models it will be possible to study the relationship between endogenous circadian systems and ultradian GH rhythms.