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Incorporation of C14 from acetate-1-C14 into lipid and CO2 by epididymal fat from golden hamsters kept at room temperature, acclimated to 5 ± 1 C, in hibernation and arousing from hibernation, was measured in vitro at 37 C. Summer and winter series were compared. The C14O2 production by tissue from control and acclimated animals was similar but the C14O2 production of tissue from hibernating and arousing hamsters was significantly greater than that from acclimated animals. There was a large increase in the lipid-C14 of tissue from cold-acclimated animals and this increase persisted into hibernation but was slightly depressed in tissue from arousing animals. Many acclimated and all hibernating hamsters had involuted testes and a greater incorporation of C14 into lipid than those with noninvoluted testes. A greater percentage of hamsters hibernated in the cold room during the winter and at this time the incorporation of C14 into lipid by the fat pad was greater than in the summer.

The percentage of glucose-U-CI4 and palmitate-I-C1a converted to C1402 .by tissue slices from golden hamsters exposed to cold, hibernating, and arousing from hibernation was studied. Liver, kidney, and diaphragm were the tissues selected. It was found that liver slices had an increased capacity to oxidize palmitate and a decreased capacity to oxidize glucose to COz throughout cold exposure, hibernation, and arousal. Kidney also had an increased oxidation of palmitate a t 48 hours of cold exposure, but this declined on cold acclimation. During hibernation, ita vitro co~~version of both palmitate and glucose to COz was reduced. Coi~versio~l of palmitate and glucose to COr by diaphragm was depressed during hibernation. During arousal, oxidation of glucose by diaphragm was greater than that during hibernation, while oxidation of palmitate did not change. I t was concluded that the results did not support a view that there is a preferential catabolization of lipid by all tissues in the cold-acclimated and hibernating hamster.

The in vitro conversion of acetate-1-C14 to C14O2 and C14-lipid by the interscapular and cervical brown fat depots of cold-exposed golden hamsters was measured. Tissue samples were taken from animals after 48 hours, 3 weeks, and 6–8 weeks in the cold, in hibernation, and arousing from hibernation and immediately after arousal. There was a depression in C14O2 production by cervical tissue after 48 hours in the cold, and by interscapular tissue after 3 weeks in the cold. C14O2 production by both depots remained low throughout acclimation, hibernation, and arousal. C14-Lipid production by both depots increased after 48 hours in the cold and remained high during acclimation. C14-Lipid production was depressed during hibernation and arousal, with recovery to acclimated levels at the end of arousal in the interscapular, but not in the cervical depot. Cervical brown fat had a higher conversion to both C14O2 and C14-lipid than interscapular brown fat. Qualitatively, but not quantitatively, brown fat behaved similarly to white fat. It was concluded that increased lipogenic capacity is not a primary response of brown fat to cold exposure of the animal, but that some other pathway becomes highly active.

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