Efferent nerve activity was recorded from pelvic nerve filaments to the urinary bladder of the cat. At intravesical pressures below 5-7 cm H2O there was no activity, but at higher pressure levels the parasympathetic neurones discharged in 10- to 15-second-long rhythmic bursts separated by silent periods. The frequency of the burst varied from about 1/min at 5 cm H2O to 6-7/min at 15-20 cm H2O. Each burst of activity was followed by a small bladder contraction. The nervous activity preceded the contraction by several seconds, indicating that the bladder response was caused by the efferent discharge. Prolonged continuous efferent activity could occur immediately after pressure changes, at pressures above 20-25 cm H2O or after inhibition caused by intravaginal stimulation. In these situations, rhythmic activity resumed as inhibition of the tonic discharge. This behaviour indicates that the rhythmicity is primarily determined by a central inhibitory mechanism. The findings are discussed in relation to the normal function of the neuronal feedback systems of the bladder and their possible role in the development of the clinical condition 'unstable bladder'.