Exposure of the urinary bladder of rats to chemical irritants such as turpentine produces a long lasting inflammatory response. Chemosensitive primary afferent fibres supplying the bladder are excited by the irritants. Dorsal horn cells in L6, S1 are also excited, and show slowly developing and persistent increases in ongoing activity, responses to electrical stimulation of vesical afferents and distension of the bladder. These changes may provide an explanation for the sensory and reflex disturbances that occur in cystitis in man.