In female rats ovariectomy (OVX) on the morning of diestrus day 2 resulted in a prompt (within 8 h) and gradual increase in LH release, whereas a similar operation on estrus did not raise serum LH until 24 h later. In proestrous females, OVX in the morning neither prevented the anticipated LH surge on that evening, nor increased LH release on the next morning. Since circulating concentrations of estradiol (E2) are progressively increased during diestrus day 2 and proestrus, this pattern of E2 secretion may affect the acute increase in LH secretion following OVX. To test such a hypothesis, we examined the effects of large, sustained or large, transient increases in circulating estrogen on the subsequent increase in gonadotropin secretion after OVX. On the morning of diestrus day 1 a subcutaneous injection of 20 µg estradiol benzoate (EB) produced prompt and large increases in both serum E2 and estrone (E1) for about 2 days, despite OVX of females at 24 h after the EB injection. An injection of 20 µg 17β-estradiol (E2) also promptly increased both serum E2 and E1. However, the magnitude of the increases in E2 and E1 after 20 µg E2 injection were twice that observed after 20 µg EB, and both E2 and E1 returned to low baseline values within 24 h after the E2 injection. In contrast, administration of 1 µg EB only produced small and transient rises in serum E2 and E1. Under these experimental conditions, it was the injection of 20 µg EB, not 1 µg EB or 20 µg E2, which elicited an LH surge on the next day, and subsequently suppressed the post-OVX increases in both LH and FSH release for 9–10 consecutive days. At 7 days after the injection of 20 µg EB both serum E2 and E1 had returned to low levels for at least 4 days in OVX (6 days) females, but the plasma concentrations of both LH and FSH were significantly less in the EB-treated than oil-injected females, indicating a sustained inhibition of gonadotropin secretion by estrogen. However, two intravenous injections of gonadotropin-releasing hormone (GnRH) elicited similar increases in both LH and FSH release between these two groups of OVX females. These observations are compatible with the hypothesis that a sustained and large rise in circulating E2 and/or E1 may exert a profound inhibition on hypothalamic GnRH release, thereby decreasing pituitary secretion of go