The amount of estradiol benzoate with progesterone required to induce lordosis in ovariectomized hamsters was determined to compare the responsiveness of hamsters to estradiol benzoate with that of rats and guinea pigs. In addition, the uptake and metabolism of tritiated estradiol in ovariectomized rats, guinea pigs, and hamsters was examined in an attempt to correlate species differences in behavioral sensitivity to estradiol with possible differences in neural affinity for the steroid. A dose of nearly 90 mg/kg was required to induce lordosis in 100% of the hamsters compared with the 2-5 mcg/kg which is effective in rats and guinea pigs. In all 3 species, highest uptake of estradiol was in the uterus and anterior pituitary gland. In the rat and guinea pig brains, the hypothalamus took up more estradiol than either the cortex or midbrain. In the hamster, there were no consistent differences in brain uptake. The affinity of the uterus, anterior pituitary, and hypothalamus of rats and guinea pigs for estradiol was greater than that of hamsters. In all 3 species, estrone was the principal metabolite of estradiol found in the tissues. The authors suggest that the higher the endogenous levels of a steroid, the less sensitive the animal is to that steroid.