The diurnal pattern of growth hormone (GH) serum levels depends on the frequency and amplitude of GH secretory events, the kinetics of GH infusion into and clearance from the circulation, and the feedback of GH on its secretion. We present a two-dimensional linear differential equation model based on these physiological principles to describe GH diurnal patterns. The model characterizes the onset times of the secretory events, the secretory event amplitudes, as well as the infusion, clearance and feedback half-lives of GH. We illustrate the model by using maximum likelihood methods to fit it to GH measurements collected in twelve normal, healthy women during 8 hours of scheduled sleep and a 16-hour circadian constant routine protocol. We assess the importance of the model components using parameter standard error estimates and Akaike's Information Criterion. During sleep, both the median infusion and clearance half-life estimates were 13.8 minutes, and the median number of secretory events was 2. During the constant routine, the median infusion half-life estimate was 12.6 minutes, the median clearance half-life estimate 11.7 minutes and the median of number of secretory events was 5. The infusion and clearance half-life estimates and the number of secretory events are consistent with current published reports. Our model gave an excellent fit to each GH data series. Our analysis paradigm suggests an approach to decomposing GH diurnal patterns that can be used to characterize the physiological properties of this hormone under normal and in pathological conditions.