Seasonal anestrus in ewes results from an increase in response to the negative feedback action of estradiol (E₂). This increase in the inhibitory effects of E₂ is controlled by photoperiod and appears to be mediated, in part, by dopaminergic neurons in the retrochiasmatic area of the hypothalamus (A15 group). This study was designed to test the hypothesis that E₂ increases multiunit electrical activity (MUA) in the A15 during inhibitory long days. MUA was monitored in the retrochiasmatic area of 14 ovariectomized ewes from 4 h before to 24 h after insertion of an E₂-containing implant subcutaneously. In six of these ewes, MUA activity was also monitored before and after insertion of blank implants. Three of the 14 ewes were excluded from analysis because E₂ failed to inhibit LH. When MUA was recorded within the A15, E₂ produced a gradual increase in MUA that was sustained for 24 h. Blank implants failed to increase MUA in the A15 area, and E₂ did not alter MUA if recording electrodes were outside the A15. These data demonstrate that E₂ increases MUA in the A15 region of ewes and are consistent with the hypothesis that these neurons mediate E₂ negative feedback during long photoperiods.