Chloracyzine is phenothiazine derivative with vasodilatory activity. Chloracyzine produced a decrease in myocardial oxygen consumption accompanied by a reduction in coronary blood flow preceded by transient coronary dilatation. Chloracyzine produced an insignificant increase in arterial pressure; heart rate increased slightly in the open-chest experiments but not in the isolated heart. It is suggested that reduced oxygen uptake after chloracyzine is realized through improved efficiency in the use of oxygen.