The cardiovascular effects of halothane, fluroxene, ether and cyclopropane in man during conditions of constant body temperature and Paco2and without concomitant surgical stress were compared. During the first hour of anesthesia, all anesthetics elevated mean right atrial pressure, increased skin blood How and decreased base excess and oxygen consumption. Ether, fluroxene and cyclopropane allowed cardiac output, stroke volume, left ventricular work and stroke work and mean arterial pressure to remain near or above control (awake) values, whereas halothane decreased all these values. Longer durations of anesthesia produced cardiovascular stimulation. After four to five hours of anesthesia, mean right atrial pressure decreased from the values at one hour to or toward control values, while cardiac output, heart rate, stroke volume, left ventricular work and oxygen consumption tended to increase. Total peripheral resistance decreased, while arterial pressure remained constant or decreased at lighter levels of anesthesia. The cause of the stimulation associated with duration of anesthesia remains unknown.