1. The Q O2 of Salamandra atra and Triturus alpestris varies with season. This periodicity appears to be largely endogenous, but in autumn an exogenous cold stimulus is required for the animals to enter the winter phase. 2. Cold (10°) acclimation stimulates Q O2 of T. alpestris in spring and autumn, whereas in summer Q O2 is stimulated by warm (20°) acclimation. 3. In both species respiration of winter animals is insensitive to acute temperature increases from 5 to 15°. 4. If for several days the head region is exposed to a higher temperature than the body (or vice versa), then the respiration of T. alpestris behaves as if the whole animal had been exposed to the head temperature. In S. atra, on the other hand, the head acclimation temperature is decivise at an experimental temperature of 20°, whereas at 10° it is the body temperature that determines whole body respiration. 5. In S. atra respiration of liver homogenates and of whole animals exhibits a temperature insensitive plateau in the region of the preference temperature.