1. To test whether distension of the urinary bladder causes a consistent change in activity in efferent renal nerve fibres, bladder distension was performed in anaesthetized dogs. The carotid sinuses were vascularly isolated and perfused with blood at constant flow. Both ureters were cannulated and the urinary bladder was distended with warm Ringer solution at a steady intravesical pressure. 2. In a first series of experiments all twenty-six renal nerve fibres in eleven dogs which responded to changes in carotid sinus pressure and changes in the nature of the blood perfusing the carotid sinuses also responded to distension of the bladder. 3. In a second series of experiments, graded bladder distension over a range of pressure of 0-9.2 kPa led to a graded increase in activity in thirteen efferent renal fibres in six dogs. The magnitude of the renal nerve response was greater at low than at high carotid sinus pressure. Over a range of carotid sinus pressure of 9-30 kPa, the greatest renal nerve activity was obtained at the lower end of this range in the presence of bladder distension. 4. Thus distension of the urinary bladder resulted in the response of a consistent increase in efferent renal nerve activity, which could be graded according to intravesical pressure. The magnitude of the responses to bladder distension was affected by carotid sinus pressure.