1. Depressed geriatric patients were treated with nortriptyline (NT) for 6 weeks. The authors measured serum levels of NT and 10-hydroxynortriptyline (10-OH-NT) using a column-switching HPLC method, and examined aging effects on NT steady-state levels to NT doses (doses/kg) ratios and NT levels to 10-OH-NT levels ratios as well as clinical response and propensity for side effects. 2. There was no significant relationship between the ages and the NT serum levels to NT doses (doses/kg) ratios, the ages and the 10-OH-NT levels to NT levels ratios, or the ages and the clinical response or the %improvement of Hamilton Scores. 3. Then the authors divided the subjects into two groups: a younger group and an elderly group with the cut off age of 60. The elderly group received significantly smaller doses of NT and had significantly lower serum levels of NT. The elderly group had tendency to have lower serum levels of 10-OH-NT. However, no significant difference was found in the improvement scores or the %improvement of depression. The elderly patients did not have higher propensity for unnegligible side effects.