Plasma levels of desipramine (DMI) and the unconjugated form of its principal metabolite 2-hydroxydesipramine (OH-D) were measured under steady-state conditions in nine depressed inpatients during treatment with 75 mg DMI every 12 hr and after at least 1 wk of an increased dose of DMI (after steady state). When DMI dosage was raised after an initial steady state had been reached, the rise in plasma DMI level was proportionately greater than the increase in dosage, suggesting saturation of DMI elimination pathways. Levels of OH-D rose in proportion to dose, suggesting saturation of DMI elimination by 2-hydroxylation could not explain DMI plasma level changes. In contrast, there were no dose-dependent effects on the disposition of amitriptyline or its metabolite nortriptyline in subjects receiving the same amitriptyline dose.