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To study the hormonal changes during hibernation in the black bear (Ursus americanus), 2 adult male bears were injected with insulin and with glucagon during their active and hibernating phases, and plasma urea, glucose, insulin, glucagon, and corticosteroids were measured before and after. The baseline urea values decreased during hibernation a pattern consistent with protein conservation. Baseline insulin values increased during the fall active phase, when bears are hyperphagic, returning to the normal range during early hibernation. Baseline glucagon levels increased during the fall hyperphagia phase and early hibernation and then tended to decrease at the end of hibernation. Baseline corticosteroid levels were lower during the summer active phase than during the other three periods. The insulin, glucagon, and corticosteroid responses to glucagon and insulin injection were variable, but in general were delayed during early hibernation. The plasma glucose response to insulin stimulation was also delayed during early and late hibernation but more so during early hibernation. The glucose response to glucagon stimulation was delayed to similar degrees during both early and late hibernation. These findings are consistent with decreased glucose utilization and increased lipolysis during hibernation. Furthermore, the apparent increase in glucose utilization at the end of hibernation when fat stores are nearly exhausted suggests a continuum of metabolic activity from early to late hibernation with a transition to the active phase by the end of hibernation.