The neuropeptide corticotropin-releasing factor (CRF) is thought to mediate the induction of a constellation of behavioral, endocrine, and autonomic responses which are important for an animal's adaptation to stressful events. We have found that the anteroventral periventricular preoptic nucleus (AVPv) and medial preoptic nucleus (MPN) of colchicine-injected female rats contained numerous intensely stained CRF-immunoreactive neurons. The AVPv/MPN in males contained very few CRF-immunoreactive neurons per section, even in colchicine-injected animals. This sexually dimorphic population of CRF-immunoreactive neurons in the AVPv may play some role in the sex-related differences in hormonal responses to stress and/or in the control of female reproductive events.