The cytokine interleukin-1 beta (IL-1β€) is an evolutionarily conserved molecule that was originally identified in the immune system. In addition to regulating peripheral immune responses, IL-1β€ plays an important role in mediating neural-immune interactions and regulating glial activities during healing and repair in the damaged nervous system. Active IL-1β€ is produced by interleukin-converting enzyme (ICE), a caspase thought to be involved in the induction of apoptosis. We report that, in the developing frog, Xenopus laevis, IL-1β€ and the IL-1 type 1 receptor proteins are coexpressed in specific neurons that comprise early sensory-motor circuits. IL-1β€ and IL-1 type 1 receptor proteins are colocalized in specific midbrain and hindbrain reticular cells, including Mauthner's neuron; specific cells in the trigeminal (fifth), lateral line (seventh), and vestibular (eighth) cranial ganglia; oculomotor neurons; and the primordial Purkinje cells of the lateral cerebellar auricle. In the spinal cord, Rohon-Beard sensory neurons, dorsal root ganglion cells, and primary motoneurons are immunopositive. Anteriorly, the olfactory pits, olfactory nerves, and olfactory bulbs are labeled, as are retinal cells, especially photoreceptor inner segments. With regard to the function of IL-1β€ during neural development, IL-1β€ and its type 1 receptor are present throughout the course of neural development in identifiable, long-lived neurons, such as Mauthner's neuron. These and other data suggest that IL-1β€ and its type 1 receptor may be involved in the maintenance of cell survival rather than induction of neuronal death.