In rats with hyperalgesia of the obliquus externus muscle (OE) from artificial calculosis of the ipsilateral upper ureter, changes in cell activity were studied in the ipsilateral spinal cord (T11-T12) versus control animals. In cases of hyperalgesia of high degree, in the dorsal horn (0-900 microns) the following were found: significantly higher percentages of cells driven by OE stimulation (P < 0.03) and of spontaneously active cells with OE input (P < 0.02); significantly higher frequency of background discharge of cells with OE input (P < 0.002); among cells driven by OE stimulation, significantly higher percentages of neurons with exclusively deep input (P < 0.0006) and of neurons with low mechanical threshold of activation (P < 0.03). In the intermediate region of the cord (900-1600 microns), a significantly higher percentage was found of spontaneously active cells with OE input (P < 0.009) while in the ventral horn (1600-2300 microns) no changes were detected. The results indicate that referred muscle hyperalgesia of high degree is accompanied by a state of central sensitization probably triggered by the abnormal afferent input from the visceral focus.