A systematic in vitro physiological study of isolated human detrusor muscle strips is reported. Detrusor strips were obtained at operation. Pre-operative urodynamic studies enabled specimens to be classified as normal or unstable. Comparisons were made between responses of normal and unstable detrusor strips. The results provide evidence of a previously undescribed feedback-loop operating via pre-synaptic alpha-adrenoceptors which controls detrusor contractility. Further evidence is provided suggesting that an imbalance in this system is present in unstable detrusor muscle.