Bats of temperate zones which hibernate during winter become cold-lethargic during their diurnal rest time even in summer. At the end of their nocturnal activity period they show a drop in body temperature close to ambient temperature (M. myotis, cf. Pohl, 1961). This takes place periodically even if the bats are kept in constant darkness (Eptesicus fuscus and Myotis lucifugus, cf. Menaker, 1959, 1961). The daily drop in body temperature is not simply the result of one single reduction in heat production, but seems to be a more complicated process since EKG-records during entrance into cold lethargy (Kulzer, 1967) showed increases and decreases in heart frequency which can be related to a regulatory activity of the sympathetic nervous system. Normothermy, with body temperatures at about 37°C, and even lethargy, with body temperatures close to ambient temperature, are both levels with more or less precisely regulated body temperature. Entrance into cold-lethargy may be the process during which the thermoregulatory level is changing from normothermy to lethargy, and the EKGvariations may be related to this change in thermoregulation. In order to obtain more data on thermoregulation during entrance into cold-lethargy, O2-consumption, CO2-production, and two different body temperatures were measured in Myotis myotis. One of the temperature records was taken from the interscapular region to measure thermogenic activity of the interscapular brown adipose tissue.