The oxygen consumption of cold exposed, restrained guinea pigs is significantly greater than that of cold exposed, nonrestrained controls. Similar observations have been made for the rat (Canad. J. Biochem. & Physiol. 33:654, 1955). These data strongly suggest that heat production is greater in the restrained animal than in the nonrestrained control. The hypothermia, then, accompanying restraint in the cold cannot be laid to a decreased muscular activity (muscular activity is actually increased) and a consequently lessened heat production, as suggested by some (J. Appl. Physiol. 12:214, 1958). It must be due, as demonstrated for the rat (Am. J. Physiol. 193:557, 1958), to an increased rate of heat loss. The marked physiological changes accompanying restraint should serve as a warning to the investigator who uses restraint for convenience in data collection. Submitted on July 11, 1958