In open-chest dog preparations i.v. administration of diazepam and chlordiazepoxide caused increases in coronary blood flow and cardiac output while reducing blood pressure, heart rate, and myocardial contractile force. The decrease in force was independent of the alteration in coronary flow. Arterial--coronary sinus oxygen difference narrowed and myocardial oxygen consumption decreased. These data show that both diazepam and chlordiazepoxide exert an oxygen conserving action along with an increase in myocardial oxygen delivery and suggest that these drugs may be of benefit to the patient with coronary insufficiency by a direct effect on the cardiovascular system as well as through CNS-mediated antianxiety effects.