D-serine appears to be a natural agonist at the "glycine site" of the NMDA receptor and is created by conversion from L-serine by serine racemase. This racemase has been localized to protoplasmic astrocytes that ensheath synapses and modulate neuronal activity in the CNS, but serine racemase expression in the PNS has not been reported. Immunofluorescence indicated that Schwann cells and other endoneurial components of rat spinal nerve contain serine racemase, and western blot analysis detected the enzyme in lysates of sciatic nerve. Cultures from sciatic nerve contained Schwann cells and fibroblasts, and both cell types showed serine racemase expression by immunofluorescence and Western blot; the quantities per unit of total protein appeared slightly lower than that expressed in cultured astrocytes. Cultures enriched for each cell type were subjected to reverse transcriptase polymerase chain reaction, further confirming serine racemase mRNA in Schwann cells and fibroblasts. Finally, immunodetection of D-serine itself was observed in cultured Schwann cells and fibroblasts. These expression patterns of serine racemase may indicate roles for D-serine in peripheral nerve transduction.