The cardiopulmonary effects of the antidepressive agent maprotiline were investigated in the dog. In ventilation experiments on dogs anaesthetized with chloralose, respiratory minute volume decreased by 5-10% shortly after intravenous injection of 1 and 3 mg/kg; acid-base-balance variables remained unchanged. After an intravenous dose of 10 mg/kg ineffective hyperventilation was observed, associated with a transient decrease of P-O2 and a slight increase in P-CO2 in the arterial blood. In cardiac-catheterization experiments on dogs anaesthetized with chloralose and morphine and receiving artificial respiration, an intravenous dose of 3 mg/kg maprotiline had no appreciable effect on the cardiovascular system, apart from a transient and slight, but statistically significant increase in pulmonary capillary pressure immediately after the injection. Anaesthesia was slightly deeper and more prolonged after the intravenous administration of maprotiline.