Cardiopulmonary consequences of acepromazine (0.2 mg/kg of body weight, IV) followed by IV administration of ketamine (10 mg/kg) were evaluated in 13 dogs. Acepromazine caused significant decreases in arterial blood pressure, stroke volume, left ventricular work, left ventricular stroke work, breathing rate, minute ventilation, and oxygen consumption. Subsequent administration of ketamine caused significant increases in heart rate, effective alveolar volume, alveolar-arterial Po2 gradient (transient increase), venous admixture (transient increase), and PaCO2 and PVCO2 (transient increases), and caused significant decreases in stroke volume, minute ventilation, physiologic dead space, and arterial and venous PO2 (transient decreases).