Golden hamsters and ground squirrels were exposed to cold air and to bodily hypothermia in a number of ways in order to find what imposed conditions would adapt them to 'cold.' Two criteria of adaptedness were decreased rate of deep-body cooling and increased resting heat production. The colon and esophagus cooled more slowly even after only one previous cooling. The difference of rate was limited to the initial portion of cooling while the body was still warm and was producing heat more actively. For induction of adaptation in hamsters a few hours of gradual cooling of both core and skin were more effective than prolonged exposure to cool air without deep hypothermia, or than head cooling without deep hypothermia or than sudden deep hypothermia. The most adequate stimulus to adaptation, therefore, was that which impinged during moderate general hypothermia. The sensitivity to adapting influences was unrelated to absolute rates of cooling or to abilities of the particular species to self-rewarm. Hamster, squirrel and rat showed similar kinds of physiological modifications in the cold-adapted state. Submitted on February 13, 1956