No sex differences were found in the capability of estradiol-17 beta (E2) to induce female sexual behaviors in adult gonadectomized rats when it was administered by a pulsatile schedule and followed by progesterone. Administration of E2 in constant-release implants resulted in a clear behavioral sex difference, with females showing higher levels than males. The ovaries of proestrous females were shown to secrete E2 in a pulsatile manner and the schedule of E2 administration which cancelled the behavioral sex difference produced pulses of serum E2 of an amplitude which was within the physiological range. Thus, the sex difference in behavioral E2 sensitivity is dependent upon the manner of E2 administration and if a physiological schedule of E2 administration is employed the sex difference is cancelled.