The effect of prolonged halothane anesthesia on myocardial oxygen uptake and coronary blood flow is unknown. This 5.5-hr study was undertaken to determine whether myocardial changes occur in dogs during prolonged steady-state halothane anesthesia. Hourly data were collected beginning 1.5 hr after induction of steady-state anesthesia. When compared to values obtained at 1.5 hr of halothane anesthesia, no significant myocardial changes were observed during the initial 3.5 hr. However, after 4.5 hr, mean arterial blood pressure increased 14% (P less than 0.01), coronary sinus flow increased 22% (P less than 0.05), and myocardial oxygen uptake increased 19% (P less than 0.05). At 5.5 hr mean arterial pressure was 18% (P less than 0.01), coronary sinus flow 31% (P less than 0.01), and myocardial oxygen uptake 21% (P less than 0.05) above levels measured at 1.5 hr. At 5.5 hr whole body oxygen uptake was 6% (P less than 0.01) above the 1.5 hr value. Cardiac output, heart rate, and systemic and coronary vascular resistances did not change significantly. This study demonstrates that duration of anesthesia is an important factor in determining the metabolic oxygen requirements of the heart. During prolonged anesthesia, the increase in myocardial oxygen demands may have an unfavorable effect on the myocardial oxygen supply-demand relationship.