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Metabolism of the essential amino acid leucine was studied in 3 black bears (Ursus americanus) in February while the bears were denning and not eating or drinking and in May while they were eating and drinking normally. (U-^14C) leucine was injected as a bolus intravenously, and timed samples of venous blood were obtained over an 8-day period. Total activity in plasma gradually increased in February and May, reaching a maximum at 12 hours. Total activity then gradually decreased to 50% of maximum after 8 days. Free 14C leucine in plasma disappeared after 8-12 hours. Although free 14C leucine was in plasma, traces of labeled glucose, pyruvate, and lactate were detected. In plasma proteins, activity was detected in the 1st sample at 30 minutes and gradually increased, describing a pattern similar to plasma total activity. Hydrolysis of plasma protein revealed that all counts were found in leucine. All 14C counts after 12 hours could be accounted for in labeled leucine. Leucine in summer and winter is rapidly removed from the plasma and incorporated into plasma proteins. Very small quantities of leucine are oxidized. Although plasma leucine is oxidized in winter, its level in blood plasma remains constant and similar to summer values. The data suggest that bears may synthesize this essential amino acid during dormancy.