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Previous studies on thyroid hormones in hibernating bears have used very few sampling periods, so that the time course of any change is poorly understood. In this study, plasma sampled from pregnant and nonpregnant black bears before and during hibernation (16 samples each at 10-day intervals) was assayed by radioimmunoassay for concentrations of thyroxine (T4) and triiodothyronine (T3). Only free T4showed a difference (P=0.019) between females that produced cubs and those that did not, but this appeared to be due to higher preimplantation values. Free T3, total T3, and free T4varied (P=0.001, 0.038, 0.002, respectively) among sampling periods: during December, bears had depressed concentrations. These lowered concentrations were maintained during hibernation for the free hormones. Our data confirm previous work showing that food restriction and/or physiological preparation for hibernation is coincident with depressed plasma concentrations of thyroid hormones. Hormonal changes associated with pregnancy were minor.