The contribution of the activity of afferent fiber filaments to pain and hyperalgesia after administration of a plantar injection of interleukin-1 beta (IL-1 beta) to the hind-paw skin was investigated by recording action potentials of the rat dorsal root in response to mechanical and thermal stimuli. Touch stimuli were delivered by stroking with a cotton-tipped applicator and thermal stimulation was applied by cooling or heating of the skin. After the administration of IL-1 beta (100 pg-1 microgram), responses to touch, cold, and heat stimulation increased to 143%, 200%, and 392%, respectively, of control values on average. IL-1 beta induced transient spontaneous discharge in 50% of experiments. The effects of IL-1 beta were apparent within 1 min. To examine responses to pressure stimulation, an area of 1 mm2 of the hind-paw skin was pressed by a mechanical stimulator. IL-1 beta (0.1 pg-200 ng) decreased the threshold value to 58% of the control pressure required for firing. IL-1 beta also increased responses to various levels of pressure (range: 1-20 g/mm2). These data suggest that IL-1 beta may play an important role in cutaneous hyperalgesia by activating polymodal receptors to mechanical and thermal stimulation.