Adult rat sciatic nerves contain cytoskeletal peptides that resemble CNS glial fibrillary acidic protein (GFAP) in immunoreactivity and molecular weight. Immunohistological examination of teased nerve fascicles indicated that these peptides are expressed selectively by Schwann cells related to small axons. Radiolabelled mouse and rat CNS GFAP cDNA probes hybridized with a single, 2.7 kb RNA band in Northern blots prepared from total RNA from both rat sciatic nerve and rat brain. Sciatic nerve GFAP mRNA was detectable by this means in adult, 2 month, or 21 day postnatal rats, but not in 3,6, or 10 day postnatal rats. Sciatic nerve transection caused a marked reduction in the level of GFAP mRNA in the axotomized distal stump. We conclude that Schwann cell synthesis of GFAP is developmentally regulated and that Schwann cells, unlike astroglia, require continued trophic input from small axons in order to express GFAP.