O2 consumption and CO2 production were measured in lean and obese Zucker rats under these conditions: thermal neutrality (acclimation and test at 25 degrees C), acute cold (acclimation to 25 degrees C and test at 15 degrees C), and chronic cold (acclimation and test at 15 degrees C). The respiratory quotient (RQ) of ad libitum-fed animals was 0.92 at neutrality, 0.87 at acute cold, and 0.77 at chronic cold. Lean and obese animals had comparable RQs under the three conditions. These results confirm previous investigations on the effect of ambient temperature on gaseous exchange in rats but fail to demonstrate a difference in substrate utilization in the metabolism of lean and obese Zucker rats.