The use of the calcium channel-blocking agent, verapamil, in beta-blocked patients has been the subject of intense investigation, particularly because both verapamil and the beta-blockers can produce negative inotropic effects. We studied the hemodynamic effects of verapamil in beta-blocked dogs to establish specific measurements that could be used clinically for early identification of combined negative inotropism. Seven anesthetized, mongrel dogs were beta-blocked with propranolol, and then given 2.5-, 5.0-, and 10.0-mg iv boluses of verapamil. The 2.5- and 5.0-mg boluses represent clinical doses, whereas the 10.0-mg bolus is a large pharmacologic dose. Hemodynamic measurements showed that verapamil was well tolerated at clinical doses; increases in stroke volume compensated for decreases in mean arterial pressure. At high doses of verapamil this response was not observed and left ventricular stroke work decreased. Cardiac and stroke indices were not useful indicators of combined drug toxicity in this dog model.