The authors investigated acute cardiopulmonary effects of noradrenaline and isoproterenol infusion in a canine model of increased pulmonary vascular resistance (PVR) and decreased cardiac output (CO). In six anesthetized, ventilated dogs, autologous blood clots were injected over approximately two hours to increase right ventricular (RV) afterload and decrease CO. After CO had decreased 40 percent dogs were treated with noradrenaline or isoproterenol in alternate sequence. Both drugs increased stroke volume but only isoproterenol affected CO. Flow increased from 1.3 to 3.0 L X min-1 (p less than .01) with isoproterenol infusion. Corresponding to the increase in CO, RV filling pressure and PVR decreased, from 9 to 5 mm Hg, and from 36 to 16 mm Hg X L-1 X min (p less than .01) respectively. When a moderate decrease in CO complicates an acute increase in PVR, isoproterenol may be an excellent drug to treat the decrease in flow.