Maximum oxygen consumption (Vo(2)) elicited by swimming in 20 degrees C water or by exposure to -2.5 degrees C in helium-oxygen (Helox) atmosphere is higher in mice selected for low (LA) than for high (HA) stress-induced analgesia (SIA) produced by swimming. However, this line difference is greater with respect to swim- than to cold-elicited Vo(2). To study the relationship between the analgesic and thermogenic mechanisms, we acclimated HA and LA mice to 5 degrees C or to daily swimming at 20 or 32 degrees C. Next, the acclimated mice were exposed to a Helox test at -2.5 degrees C and to a swim test at 20 degrees C to compare Vo(2) and hypothermia (DeltaT). Cold acclimation raised Vo(2) and decreased DeltaT. These effects were similar in both lines in the Helox test but were smaller in the HA than in the LA line in the swim test. HA and LA mice acclimated to 20 or 32 degrees C swims increased Vo(2) and decreased DeltaT elicited by swimming, but only HA mice acclimated to 20 degrees C swims increased Vo(2) and decreased DeltaT in the Helox test. We conclude that the between-line difference in swim Vo(2) results from a stronger modulation of thermogenic capacities of HA mice by a swim stress-related mechanism, resulting in SIA. We suggest that the predisposition to SIA observed in laboratory as well as wild animals may significantly affect both the results of laboratory measurements of Vo(2) and the interpretation of its intra- and interspecific variation.