An animal model of muscular hyperalgesia was developed. In humans, this disorder follows painful crises due to ureteral calculosis. Changes in vocalization thresholds to electrical stimulation of the obliquus externus muscle of both sides were studied in a group of rats with chronically implanted muscles before and after the production of a stone in one ureter. In another group of rats with implanted muscles, it had been verified preliminarily that these thresholds did not vary widely from day to day. On the contrary, a significant lowering in threshold (max 31%) in the muscle ipsilateral to the implanted ureter appeared the day after the production of the stone and persisted for the subsequent 10 days of observation, although less pronounced during the last 5 days. Hypersensitivity to manual pressure was also observed, mainly in the ipsilateral muscle, in most rats during the same period. In order to differentiate between the effects due to the presence of the stone in the ureteral lumen and those due to the spontaneous occlusion which frequently occurred in the implanted ureter, 2 other groups of rats were studied. In one, a unilateral ureteral ligature was performed; in the other, the production of a stone was combined with a ligature (placed distally to the stone). Ligature alone never induced any hyperalgesic effect. Stone plus ligature produced a marked hyperalgesia (max 39%) in the ipsilateral muscle, which lasted for only 5 days. It is concluded that the ureteral stone is the factor responsible for the appearance of muscular hyperalgesia.