The multifunctional cytokine interleukin 6 (IL-6) has direct growth, survival and differentiation effects on peripheral and central neurons. Furthermore, it can modulate the production by non-neuronal cells of other cytokines and growth factors, and thereby affect nerve cells indirectly. We have studied IL-6 expression and production in intact and injured peripheral nerves of C57/BL/6NHSD mice, which display the normal rapid progression of Wallerian degeneration. The IL-6 mRNA was detected in nerves degenerating in vitro or in vivo, but not in intact nerves. In vitro- and in vivo-degenerating nerve segments and neuroma nerve segments synthesized and secreted IL-6. The onset of IL-6 production was rapid and prolonged. It was detected as early as 2 h after injury and persisted for the entire period of 21 days tested after the injury. Of the non-neuronal cells that reside in intact and injured nerves, macrophages and fibroblasts were the major contributors to IL-6 production. We also studied IL-6 production in intact and injured nerves of mutant C57BL/6-WLD/OLA/NHSD mice, which display very slow progression of Wallerian degeneration. Injured nerves of C57BL/6-WLD/OLA/NHSD mice produced significantly lower amounts of IL-6 than did rapidly degenerating nerves of C57/BL/6NHSD mice.