Cortisol and ACTH exhibit circadian rhythmicity, peaking in the early morning. These peaks are associated with increased activity and alertness. We sought to determine whether self-reported daily rhythms predict outcome of a.m. and p.m. CRH challenge in elderly subjects. We surveyed 96 elderly subjects to determine daily rhythms in activity levels, mood, alertness, and performance. Seven healthy subjects were given a cumulative activity score reflecting propensity toward morningness or eveningness. Subjects underwent CRH challenge testing during the morning and evening hours of different days. Baseline plasma ACTH and cortisol concentrations were higher in the morning than in the evening and lower values were associated with lower activity scores (i.e., greater morningness). No trends were apparent between activity score and net hormone response or percent change in hormone concentration.