PURPOSE: To compare the physical characteristics of detrusor overactivity (DO) induced by intravesical infusion of saline in awake, sham rats and rats with chronic spinal cord injury (SCI), by simultaneous registrations of intravesical and intraabdominal pressures. METHODS: Male Sprague-Dawley rats, normal or with a spinal vascular clip at the level of Th9, were investigated cystometrically 1 and 4 weeks after SCI. Intra-vesical pressure (IVP) and intra-abdominal pressure (IAP) were recorded simultaneously to evaluate true DO. During the filling phase, the event of IVP rises, defined as increments that exceeded 2 cmH2O from baseline, were determined as DO according to the absence of simultaneous changes in IAP. RESULTS: All SCI rats exhibited DO during the filling phase, which was not shown in sham rats. The frequency and pressure of DO had a tendency to decrease with time. The DO frequency of SCI rats after 4 weeks (0.9+/-0.2 min(-1)) was decreased compared with that after 1 week (2.1+/-0.4 min(-1); P<0.05). The DO pressure of SCI rats after 4 weeks (8.4+/-1.9 cmH2O) was decreased compared with that after 1 week (11.6+/-2.9 cmH2O; P>0.05). CONCLUSIONS: Cystometric studies in awake male SCI rats showed some significant changes in bladder function after SCI. All SCI rats exhibited DO during the filling phase, and showed different physical characteristics of DO over the course of time. The neurological basis of these time-related changes remains poorly understood, but may provide important prognostic information about long-term urological management in SCI patients.