Steady-state serum concentrations of desipramine (DMI) and its metabolite, 2-hydroxydesipramine (OHDMI), were measured in 40 children, 36 adolescents, and 27 adult psychiatric patients. The authors predicted that younger patients would show more efficient elimination of DMI, with greater amounts of OHDMI. OHDMI averaged 52% lower than DMI. DMI per weight-corrected dose (ng/mL: mg/kg) rose significantly with maturation, from 50 in children and 56 in adolescents to 91 in adults. Contrary to expectation, OHDMI per DMI dose also rose with age, from 17 in children and 20 in adolescents to 26 in adults. It was concluded that: (1) similar mg/kg doses of DMI result in lower DMI and OHDMI in children; (2) children metabolize both DMI and OHDMI more rapidly than adults; and (3) children do not have high circulating concentrations of OHDMI.