The in vivo rate of gluconeogenesis in the male rat was studied during acute exposure and acclimation to a cold environment (5 ± 2°), using an isotopic-tracer method. Four key gluconeogenic-enzyme activities (pyruvate carboxylase (EC 6.4.1.1), phosphoenolpyruvate carboxykinase (EC 4.1.1.32), fructose 1,6-diphosphatase (EC 3.1.3.11), and glucose 6-phosphatase (EC 3.1.3.9)) were compared with the in vivo rates of gluconeogenesis. Phosphoenolpyruvate carboxykinase showed the largest increase in activity in the cold (175%) but this was much smaller than the maximum increase in gluconeogenic rate in cold-stressed rats (485%). Although qualitative similarity between changes in enzyme level and the gluconeogenic rate is evident, a close correlation does not exist. Gluconeogenesis is more active during acute cold exposure (1–7 days) than in chronic cold exposure (3 weeks).These data are interpreted to indicate that the cold-stressed rat which is shivering probably has an accelerated Cori cycle (glucose degradation to lactate in the muscle and resynthesis in the liver) which increases the energy supply (ATP) in the muscle and contributes a minor amount towards thermogenesis in the cold-stressed rat. This contribution diminishes as the capacity for nonshivering thermogenesis in the rat develops.