Chronic decerebrate rats, maintained in good condition for 31-84 postoperative days, showed significant within-session habituation of the acoustic startle response. However, they showed no habituation over days under conditions that produced significant response deficits in controls. The decerebrates' stimulus-provoked response deficits may have endured for up to 40 min between sessions, and they were apparently more than normally susceptible to stimulus-provoked sensitization. The data are consistent with intrinsic models which assume that the mechanisms for short-term habituation are intrinsic to the stimulus-response (S-R) pathway in the lower brain stem. On the other hand, relatively permanent long-term habituation of the acoustic startle response must be mediated by extrinsic mechanisms rostral to the S-R pathway which superimpose an inhibitory influence on that pathway.