Distension of the perfused guinea pig ureter at pressures from 20 to 700 cmH₂O increased the amount of ATP released from the epithelium in a pressure-dependent manner. During basal perfusion (40 μl/min), the perfusate contained 10 pmol/ml ATP; this increased 10- to 50-fold at various distending pressures. ATP was released from epithelial cells during distension as mechanical removal of the urothelium blocked release. No lactate dehydrogenase was detected in the perfusate, and scanning electron microscopy confirmed an intact urothelium after distension. ATP was not released due to the activation of stretch-activated channels, as gadolinium (10 μM) failed to affect ATP release. Glibenclamide (10 μM), known to inhibit two members of the ATP-binding cassette (ABC) protein family, did not affect ATP release after distension; nor did verapamil (10 μM). In contrast, both monensin (100 μM) and brefeldin A (10 μM), which interfere with vesicular formation or trafficking, inhibited distension-evoked ATP release, which was Ca²⁺-dependent. This suggests that ATP release from the ureter epithelium might be mediated by vesicular exocytosis. The role of ATP released by distension of hollow visceral organs is discussed in relation to the concept of purinergic mechanosensory transductions, with special reference to nociception and the activation of P2X₃ receptors on the subepithelial sensory nerves.