1. The present study has investigated the receptive properties of myelinated mechanoreceptive primary afferents innervating the inflamed urinary bladder of the cat. In 15 experiments, 20 units were recorded from the dorsal and 3 from the ventral root S2. Before inflammation the afferents had no resting activity and responded consistently to increases of intravesical pressure evoked by isotonic distension or isovolumetric contractions. All units were studied before and after the onset of an acute inflammation induced by intraluminal injection of mustard (1-2.5%) or turpentine oil (50%), which are known to induce an acute cystitis. 2. Eleven out of 14 units tested with mustard oil and 5/9 units tested with turpentine oil were activated at short latency. The response could not be explained by a concomitant increase of intraluminal pressure resulting from the intravesical injection of the irritant. This suggests that a large proportion of mechanosensitive afferents has an additional chemosensitivity. 3. After removal of the irritants and with empty bladder, all afferent units exhibited irregular ongoing activity with intermittent high-frequency bursts. Such ongoing activity was entirely absent in myelinated afferents supplying the noninflamed bladder. The median rate of ongoing activity was significantly higher after mustard oil (1.65 imp/s) than after turpentine oil treatment (0.05 imp/s) 1 h after chemical stimulation. Post-hoc analysis revealed that afferents that developed high levels of ongoing activity had steeper stimulus response functions to changes of intravesical pressure before inflammation. 4. The stimulus-response function of vesical afferents changed characteristically in the inflamed bladder. Within 30 min of mustard oil treatment, the responses of some units to bladder filling was transiently enhanced, but later the units desensitized to this stimulus. However, there was no significant change of the stimulus-response function of six afferents studied before and for 2 h after induction of the inflammation. By contrast, the afferents exhibited significant increases of their mechanosensitivity after turpentine treatment. 5. In conclusion, sacral myelinated mechanoreceptive afferents supplying the urinary bladder are chemosensitive. Their receptive properties change rather uniformly when the tissue becomes inflamed, and, on the basis of these changes, it is not possible to differentiate subpopulations of primary afferents. The induction of ongoing activity and the increased responses to intravesical pressure stimuli in the inflamed organ, together with central changes that may occur under these conditions, are probably responsible for changes of bladder motility and sensations that arise during cystitis.