In the skin activation of unmyelinated primary afferents produces vasodilation and plasma extravasation. Here, using Evans blue dye leakage, we have quantitatively compared the ability of mechanical, chemical and electrical stimulation of afferents to produce extravasation in the urinary bladder of anaesthetized rats. Mechanical stimulation - a series of large active contractions of vesical smooth muscle induced by maintained distension - elicited no increase in extravasation compared to controls. In contrast, a similar period of electrical stimulation of nerves supplying the bladder, or chemical stimulation with intravesical instillation of mustard oil, produced high levels of extravasation. We conclude that afferents activated during distension and micturition contractions are different from those which produce extravasation. The latter may be important in pathophysiological cases such as inflammation of the urinary bladder.