Mild psychological stressors provoke an acute rise in core temperature (T_c), stimulate the hypothalamo-pituitary-adrenocortical (HPA) axis and induce various stress-related behaviors. In the present study we examined the effect ablation of the anteroventral third ventricle region (AV3V) had on both physiological and behavioral responses to a novel environment. T_c was monitored in male Sprague-Dawley rats, with either sham or AV3V lesions, during a 5 h exposure to a novel environment. Trunk blood was collected, in a second group of rats, for the assessment of plasma levels of ACTH and corticosterone. Novelty-induced grooming and rearing behaviors were assessed in a third group of animals. T_c was elevated in all animals after 30 min in the novel environment, but the rise was exaggerated in rats with AV3V lesions (~0.5°C). AV3V-lesion rats maintained a higher core temperature for 2 h before it returned to the same level as the control group. Plasma levels of ACTH and corticosterone were also exaggerated in the AV3V-lesion group after 30 min in the novel environment. In contrast to the physiological responses the behavioral measures of grooming and rearing revealed no differences between the groups. The results from the current study suggest that neurons within the AV3V region exert an inhibitory influence on the HPA axis and fever developed in response to stressful psychological stimuli. They also confirm that the physiological and hormonal components of the stress response are independent of certain behavioral measures of stress.