To study the relative importance of neurogenic factors in detrusor contractility, active bladder wall stress values were compared in situ and in vitro. Eight male guinea pigs were used. The active stress in the bladder wall in spontaneous micturition contractions was calculated from the results of urodynamic examinations and compared with the active stress developed in response to optimum electrical stimulation in full-thickness bladder wall strips taken from the same bladders. The results indicated that, in normal micturition, the detrusor muscle is not fully stimulated, and the rate of pressure development is not determined by mechanical factors. To identify topological variations of detrusor contractility, the strips were taken from three different locations. It was found that strips from the posterior wall contracted more forcefully than those from the anterior wall.