The expression of interleukin-lp was examined in dorsal root ganglion (DRG) neurons from adult rats using non-radioactive in Situ hybridization and immunocytochemistry. At all spinal levels, approximately 70% of the DRG neurons appeared to express IL-1 beta mRNA: about 80% of these DRG neurons actually appeared to produce the IL-lp protein at markedly varying levels. The expression of IL-1 beta was found in large as well as in intermediate diameter sensory neurons but only sporadically in the population of small sensory neurons. The population of IL-1 beta immunopositive sensory neurons included most of the large calretinin-positive la afferents. but only a few of the small substance P/CGRP positive sensory neurons. In situ hybridization staining for the detection of type 1 IL-1 beta receptor showed expression of this receptor by most of the sensory neurons as well as by supportive glial-like cells, presumably satellite cells. The functional significance of IL-1 beta in the DRG neurons needs to be elucidated, but we speculate that IL-1 beta produced by DRG neurons may be an auto/paracrine signalling molecule in sensory transmission. (C) 2001 Elsevier Science B.V. All rights reserved.