Twenty patients suffering from endogenous depression were treated with maprotiline for 4 weeks. Blood samples were collected at weekly intervals and severity of depression assessed using the Hamilton Depression Rating Scale. No simple relationship between plasma maprotiline concentration and amelioration score was observed at week 3 (rs = 0.25) or week 4 (rs = -0.05). No significant difference in plasma concentrations between responders and non-responders was observed at week 4. Maprotiline was effective as an antidepressant in some patients.