In estrogen-treated and non-treated ovariectomized female rats under urethananesthesia, 278 neurons were antidromically identified in the medial preopticand other basal forebrain areas by electrical stimulation of their terminalsin the ventral tegmental area. Responses from the estrogen-treated rats had asignificantly higher threshold, longer latency and absolute refractory periodthan those from the non-treated rats. Estrogen may decrease excitability ofthe preoptic axons along its length.