Twenty out-patients with depressive illness were studied with regard to thyroid hormone concentrations, basal and TRH-stimulated TSH, and prolactin levels before and during treatment with nortriptyline. Concentrations of plasma nortriptyline and 10-hydroxy-nortripyline were analysed while on 75 mg and 150 mg/day. No correlations between clinical outcome and side effects on the one hand and drug concentrations on the other were demonstrated. Provided the age factor was considered, basal and stimulated hormone levels did not differentiate between healthy controls and patients with "endogenous" and those with "exogenous" depression. Nortriptyline treatment did not influence serum hormone levels.