In freely moving, estrogen- and progesterone-treated ovariectomized female rats, electrical stimulation of the medial preoptic area, which had been isolated dorsally from the septum by a horizontal knife cut (roof cut), caused immediate interruption of the lordosis reflex, a principal component of female reproductive behavior in this species. Neither the knife cut nor the stimulation affected their proceptive interactions with the males. Lordosis was interrupted in a graded manner in response to increased stimulus intensity, with a threshold at 30 microA. The optimal frequency was at 50-100 Hz. Lordosis was reinstated promptly by the termination of current application. The rapid time course distinguished the stimulation effect in the rats with the roof cut from that in the intact control animals, which was slow in its onset and recovery. In the sham-operated animals, an adjunct bilateral cut of the stria terminalis was sufficient to restore the rapid response as in the roof-cut animals. The roof-cut animals were sensitive to lower doses of estrogen than those without the knife cut; therefore, the preoptic area appears to be a separate entity from the septum in the inhibitory control of lordosis. The elimination of facilitatory neural components for this reflex, which enter the preoptic area dorsally via the stria terminalis, might be responsible for the prompt and exaggerated stimulation effect in the roof-cut animals.