The urinary bladder is a compliant organ, high compliance being essential for useful urine storage. The extent to which the sympathetic nervous system promotes the storage of urine by increasing bladder compliance is unclear. The aim of the present study was to determine the range of bladder volumes over which the sympathetic nervous system increased bladder wall compliance. In supine, anaesthetized cats, the bladder was filled at twice the rate of natural filling, the continence cycle being interrupted at five stages. These stages were when the bladder had become globular, during prodromal contractions, soon after non-micturating contractions had commenced, approximately two-thirds of the way through the continence cycle and just prior to micturition. During each of these interruptions, bladder volume was held constant while pelvic nerve afferent activity and bladder pressure were recorded. Recordings were obtained before and during the intravenous infusion of trimethaphan, the resulting partial ganglion blockade decreasing arterial pressure by a third. Bladder pressure as well as afferent nerve activity increased significantly when the sympathetic drive was transiently blocked, indicating that there had been a prevailing net sympatho-inhibitory effect promoting bladder wall relaxation. This effect was observed during prodromal contractions and continued until the onset of micturition. This net sympatho-inhibitory effect is a potential therapeutic path for the treatment of bladder storage disorders.