Female hooded rats (230 to 260 g) were ovariectomized and given a subcutaneous implant of an estradiol-filled Silastic tube. The length of the tube was varied in order to produce a variety of serum estradiol levels. In the first experiment, animals were weighed over a 6-week period following surgery and then tested for sexual responsiveness to a male. The results demonstrated that ovariectomized females with an implant maintaining a serum estradiol concentration at about 15 pg/ml maintained body weight at the same level as that of intact females. A smaller implant gave rise to a higher weight gain and a larger implant to a lower weight gain. All implants resulted in a continuous state of receptivity. In a second experiment, ovariectomized females were implanted with smaller estradiol-filled implants in order to determine the threshold for maintaining proceptivity and receptivity. The results indicated that with a serum estradiol concentration below 15 pg/ml, the frequency of lordosis and of ear wiggling and darting decreased. Progesterone injections facilitated both proceptive and receptive behavior. In addition, following progesterone injections, the time required for a male to mount a female 10 times was decreased in females with low or no estradiol replacement. These results indicate that a constant concentration of estradiol at about the mean level present throughout the estrous cycle will result in normal body weight regulation and will maintain sexual behaviors that normally occur only during estrus. These results emphasize that Silastic implants of estradiol do not mimic normal endocrine function since, even at low levels, estradiol implants produce continuous receptivity.