The object of this study was to determine the influence of ovarian steroids on pulsatile LH release in the interval between the mornings of diestrus 2 (D2) and proestrus in the rat. Four groups of rats were bled continuously for 3 h between 09.30–12.30 h at a rate of 75 µl whole blood/6 min: (1) bled on D2; (2) sham ovariectomy (OVX) on D2 and bled on proestrus; (3) OVX on D2, implanted with empty or oil-filled capsules, and bled 24 h later; and (4) OVX on D2, implanted with estradiol (E2) capsules, and bled 24 h later. Between D2 and proestrus, plasma E2 levels increased from 13 ± 1 to 42 ± 9 pg/ml, and progesterone levels decreased from 27 ± 3 to 13 ± 2 ng/ml, the latter reflecting the decline of the corpus luteum early on D2. Between D2 and proestrus there was no change in mean blood LH levels, LH pulse amplitude, or pulse frequency. However OVX on D2 increased mean blood LH levels 2.5-fold over values on proestrus due to a 3.5-fold elevation in LH pulse amplitude and an 80% increase in pulse frequency. E2 levels fell in these rats to 8 ± 1 pg/ml. Restoration of physiological proestrous levels of E2 (46 ± 5 pg/ml) significantly reduced the increase in mean blood LH levels by lowering pulse frequency to proestrous values, and by causing a 50% reduction in pulse amplitude. However, LH pulse amplitude and therefore mean blood LH levels were still higher than values on proestrus. The possibility that ovarian progesterone might also regulate LH pulse amplitude in the D2-proestrous interval was ruled out since there were no significant differences in plasma progesterone levels between 13.00–19.00 h following sham OVX or OVX of rats earlier on the morning of D2. In vitro incubation studies with anterior pituitaries from rats ovariectomized 24 h prior on D2 and implanted with E2 or empty capsules demonstrated that E2 increased the pituitary sensitivity to LHRH, indicating that the E2-induced decrease in LH pulse amplitude could not be exerted at the pituitary level. In conclusion, since LH pulse amplitude and frequency remain unchanged between the mornings of D2 and proestrus, yet increase following OVX, the present data demonstrate that ovarian E2 acts centrally to exert a restraining effect on both parameters of pulsatile LH secretion during this interval in the rat est