To study the relative importance of neurogenic factors in detrusor contractility and to relate a total bladder in vitro contractility model to a previously described bladder wall strip model, active intravesical pressure values were compared in situ and in vitro in eight male guinea pigs. In situ, the active pressure was measured in spontaneous isometric and non-isometric micturition contractions. In vitro, the active pressure was measured in isometric contractions of the same bladders, developed in response to optimal electrical stimulation. The volume dependence of the active pressure generated by the bladder was measured in vitro in order to relate bladder capacity to the volume where the generated force is maximal and to determine the optimal volume at which to study detrusor contractility. The results indicated that in normal micturition the detrusor muscle was not fully stimulated: active pressure in isometric contractions in vivo was about 60% of the pressure values attained in vitro at the same bladder volume. Most micturitions occurred at a volume where the active pressure generated in vitro was about 80% of the maximal pressure. The active pressure-bladder volume relationship complied with the sliding filament-cross bridge theory. In whole bladder preparations active stress was about twice as high as in strips.