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Serum progesterone levels in 5 female Hokkaido brown bears (Ursus arctos yesoensis) were determined from April 1983 to April 1985. Four females were mated with males, while the 5th female was segregated to serve as a control. The growth of fetuses in 2 females was monitored by ultrasonography at 10-day intervals during December and January. Serum progesterone levels increased from basal levels of 0.4-1.1 ng/ml on day -300 (300 days before parturition) to higher levels at approximately day -210. The levels, which ranged from 1.6 to 5.4 ng/ml, were maintained until approximately day -60, when they sharply increased to 8.3-9.7 ng/ml. The levels then dropped on day 0 and remained at 0.4-0.5 ng/ml between day 30 and day 90. Change in serum progesterone level in the unmated female was similar to that of the mated animals. The images of fetuses in 2 bears were first observed on day -37 and day -33 when their crown-rump length was 1.5-2.0 cm. Thereafter fetus growth was sigmoidal. The females carried 2 and 3 fetuses and had 1 and 3 cubs 1 month postpartum, respectively.