In the present experiment we examined the effect of estrogen upon in vivo luteinizing hormone releasing hormone (LH-RH) release from the medial basal hypothalamus of freely moving ovariectomized female rats during the period of the LH surge. Ovariectomized females (10–20 day) received two subcutaneous injections of oil or estradiol benzoate (EB; 25 µg/rat) at 48 and 24 h prior to push-pull perfusion and were perfused between 10.00 and 18.00 h on the 3rd, day. Unexpectedly, the activity of the LH-RH pulse generator in ovariectomized rats was characterized by a very low mean LH-RH output. Administration of EB increased the activity of the LH-RH pulse generator as indicated by a significant increase in overall mean LH-RH release of EB- versus oil-treated females. In addition, EB-treated females demonstrated a diurnal variation in LH-RH release with marginal, but significant, increases in LH-RHrelease during the period of the LH surge (14.10—18.00 h) as compared with levels obtained between 10.00 and 14.00 h. This increase in the overall mean release during the period between 14.10 and 18.00 h appears to be attributable to an EB-induced increase in the frequency of LH-RH release, since the amplitude did not change between the two 4-hour sampling periods. These results indicate that administration of EB in chronic ovariectomized rats enhances the overall activity of the LH-RH pulse generator, and superimposed upon this augmented activity in the LH-RH pulse generator there is an EB-dependent diurnal variation characterized by an increase in the frequency, but not in the amplitude, of the LH-RH signal during the afternoon.