The effect of interleukin-1β (IL-1β) on the electrical properties of sensory neurons was assessed at levels and exposure times comparable to those found in animal models of neuropathic pain. Experiments involved whole cell current-clamp recordings from rat dorsal root ganglion (DRG) neurons in defined-medium, neuron-enriched cultures. Five- to six-day exposure to 100 pM IL-1β produced subpopulation-dependent effects on DRG neurons. These included an increase in the excitability of medium-diameter and small-diameter isolectin B(4) (IB(4))-positive neurons that was comparable to that found after peripheral nerve injury. By contrast, a reduction in excitability was observed in large-diameter neurons, while no effect was found in small-diameter IB(4)-negative neurons. Further characterization of changes in medium and small IB(4)-positive neurons revealed that some, but not all, effects of IL-1β were mediated through its receptor, IL-1RI. Although the acute actions of IL-1β on sensory neurons have been well studied and related to acute and/or inflammatory pain, the present study shows how sensory neurons respond to long-term cytokine exposure. Such effects are relevant to understanding processes that contribute to the onset of neuropathic pain.