Young animals exposed to cold environmental temperatures typically have decreased skeletal muscle accretion but increased heart masses. To explore these phenomena, we measured protein synthesis and degradation in vivo in cardiac and skeletal muscle in weanling rats during short-term cold exposure and rewarming. Control rats were housed at 25 degrees C throughout the experiment. Ad libitum-fed and pair-fed (to the intake of controls) rats were housed at 5 degrees C (cold) for 5 days and then at 25 degrees C (rewarmed) for another 5 days. Cold exposure decreased rates of protein accretion and synthesis in skeletal muscle, whereas degradation did not differ. The effects of cold exposure on skeletal muscle were similar in both pair-fed and ad libitum-fed rats, except growth was lower in pair-fed rats. In cardiac muscle, cold exposure increased rates of protein synthesis and degradation and resulted in increased cardiac mass. Results in pair-fed animals generally fell between those of control and ad libitum-fed cold rats. During rewarming, growth rates were not higher in skeletal muscle in ad libitum-fed re-warmed rats, although protein turnover returned toward control values; in pair-fed rats, it remained lower. In heart, growth rates of ad libitum-fed and pair-fed rewarmed rats decreased due to lower protein synthesis rates. These alterations appear to be consistent with a strategy designed to improve survival in cold environments.