The relationship between steady-state plasma concentraton and clinical effect of imipramine in the treatment of nocturnal enuresis was studied in 22 hospitalised children. After 1 week on placebo the children were given imipramine in a fixed dose of about 1 mg/kg for 3 weeks. The enuresis frequency decreased significantly from the placebo to the first week on imipramine, but then no further improvement was observed. There was a significant correlation between steady-state plasma concentration of desipramine or imipramine + desipramine and the reduction in enuresis frequency during imipramine treatment. The optimum effect was obtained when steady-state levels of imipramine + desipramine were above 60 microgram/L; i.e. the effective concentration in enuresis is 3 to 4 times lower than in antidepressive therapy. Dose- and weight-corrected, steady-state plasma concentration of imipramine and desipramine were not significantly different from those previously observed in younger adults. In 1 child, a transient rise in imipramine and desipramine concentration was seen during a period with fever and bacterial infection.