Injection of a compound algogenic substance into a receptive field of the skin induced sustained discharges from single polymodal nociceptors (PMNs) in rats. Stimulation of the sympathetic nerve (SS) innervating the receptive field obviously increased the sustained discharges. Some units were first facilitated and then inhibited. The sympathetic effect could be activated repeatedly and lasted a long time. Injection of norepinephrine (NE, 5 micrograms) into the local artery caused a similar effect. The results indicate that the sympathetic nerve could facilitate sustained discharges of PMN. The possible causes of causalgia and different results of other experiments are discussed in this paper.