This study, conducted on female black bears from 3 study areas in the Rocky Mountains, showed that pregnant females in a state of diapause during early winter had about 89% larger fat depots than did nonreproductive females going into hibernation. Fat provided 92% of the total energy for lactation and gestation. Rates of fat loss (g/day) were 37% greater and protein loss about 2.4 times higher for reproductive females than for nonreproductive females. The cost of winter reproduction, including gestation and lactation, was 1,432 kJ/day to produce 2 young. Although reproduction required elevated protein breakdown, rates of overall protein loss were relatively small, perhaps due to a short period of implantation and an extraordinary ability to hydrolyze urea.