The purpose of this experiment was to investigate whether endogenous corticotropin-releasing factor (CRF) within the paraventricular nucleus of the hypothalamus (PVN) is involved in the alteration of locomotor activity related to a novel, presumably stressful, environment in rats. Impairment of the function of CRF-containing neurons was effected by immunotargeting ricin A chain toxin to these cells with a monoclonal antibody to CRF (CRF-MAb/toxins). CRF-MAb/toxins administered into the PVN 2 weeks prior to testing produced an increase in locomotor activity during exposure to novel environment photocell cages. This behavioral activation was maintained throughout the 120 min experimental session, but was not present when the rats where introduced to the same photocell cages after extensive habituation. These results suggest that the effect induced by CRF-MAb/toxin treatment was related to the exploration of the novel environment, and was not due to a generalized locomotor hyperactivity. This effect was accompanied by a 53% and 61% CRF decrease in the hypothalamus and the median eminence, respectively. Injection of CRF-MAb/toxins into the ventromedial nucleus of the hypothalamus did not modify the locomotor activity in either unfamiliar or familiar conditions. These data suggest that CRF neurons within the PVN may participate not only in the activation of the pituitary adrenal axis associated with stressors but also as physiological mediators of the behavioral responses to stress.