Abstract Electrical stimulation of the dorsal periaqueductal gray (DPAG) or the deep gray layer of the superior colliculus (DGSC) of rats placed in an open-field elicited either a display of tense immobility, accompanied by exophthalmus and/or defecation and micturition, or running and jumping responses. Threshold curves of each response were obtained for each structure by the logistic fitting of accumulated response frequencies. DPAG and DGSC threshold curves were compared by likelihood-ratio coincidence tests. The output of micturition was significantly higher following the stimulation of DPAG ( P<0.0005). In contrast, no differences were found for the remaining responses. These data support previous studies in anaesthetized cats suggesting the critical involvement of DPAG in the control of micturition. Furthermore, they also suggest that topographically distinct neural networks within the DPAG and DGSC control micturition and the other defensive behaviors.