There is compelling evidence for feedback disturbances in the hypothalamus-pituitary-adrenal system associated with human aging as assessed by challenge tests. However, reports about age-related changes in human basal activity are ambiguous and to date little is known about changes in the pulsatile features of the HPA system. To investigate these changes we studied twenty-two healthy male and eleven healthy female subjects ranging from 23 to 85 and 24 to 81 years respectively. 24-hour blood sampling with 30 minute sampling intervals was performed. From 18.00 to 24.00 hours blood was sampled every 10 minutes for analysis of pulsatile features of HPA activity. Statistical analysis revealed that age in particular had major effects upon basal HPA-system activity: there was a significant age-associated increase in minimal (p < 0.0001) and mean (p < 0.02) cortisol plasma concentrations, but no alteration in pulsatile features. We found no age-cortisol correlation during daytime, but were able to demonstrate a strong impact of age upon cortisol plasma levels from 20.00 to 1.30 hours. The diurnal amplitude of cortisol (p < 0.005) and ACTH (p < 0.006), relative to the 24-hour mean of the hormones, showed an age-associated decline. Additionally, the evening cortisol quiescent period (p < 0.01) was shortened in the elderly, suggesting increasingly impaired circadian function in aging. Our results suggest an increased basal activity and a flattened diurnal amplitude of the HPA system in the elderly.