These experiments examined the behavioral, corticosterone, and prolactin responses to a novel environmental in male rats with total fornix transections. During the initial exposure to the novel environment, rats with fornix transections exhibited increased latency to initiate activity and increased activity levels. There were no changes in frequency of rearing, frequency of interactions with objects in the environment, or defecation. Fornix transection was found to have no effect on resting levels of either corticosterone or prolactin but to alter significantly the stress-response profiles of each of these hormones. The corticosterone responses to the novel environment in rats with fornix lesions were of greater magnitude with no change in latency to respond or in recovery of resting levels, whereas the prolactin responses were of shorter latency with no change in magnitude or recovery or resting levels. Habituating to the novel environment by repeated exposure was found to decrease the corticosterone responses of rats with fornix lesions but not the responses of controls. The results are discussed in terms of differences between rats with fornix lesions and control rats in level of arousal and effect of experience in the environment.