A study of the acute oxygen consumption of 16 young vipers (8 Vipera aspis and 8 V. berus) at temperatures of 10-15-20-25-30°C shows a number of distinctive features common to both species, and also specific differences. The oxygen consumption obviously increases with temperature, more or less irregularly for the average consumption which partly depends on the activities of each animal, while it follows an exponential progression for the minimum consumption and a linear progression for the maximum consumption. In the total consumption the part caused by the rather limited exploratory activities of our animals (routine aerobic scope) decreases as the temperature rises and it even decreases in absolute value between 25 and 30°C. At all temperatures, the oxygen consumption of V. berus is greater than that of V. aspis, the difference being globally significant. However, these differences are important only for the minimum consumption at low temperatures. These specific differences are discussed according to the ecology and the geographical distribution of the two species.