The role of microglia in the response to CNS injury is not fully understood. We characterized the temporal activation of microglia in the adult spinal cord following a lesion that severed the axons of the dorsal columns and corticospinal tract at T8. Two days after lesion, microglia in the severed T4-T5 fasciculus (f.) gracilis were ameboid and expressed intense OX42 and increased class I major histocompatibility complex (MHC) antigen (OX18) immunoreactivities. No activated microglia were seen in the intact f. cuneatus or the corticospinal tract. Five days postlesion, OX42 immunoreactivity was slightly decreased in the f. gracilis, and OX18 expression was slightly enhanced. By 12 days postlesion, OX42 and OX18 immunoreactivities were near control levels. At L1-L2, activated microglia with increased OX18 expression were restricted to the corticospinal tract and were maximal 5 days postlesion, returning to near control levels by 12 days postlesion. In the medulla, enhanced OX42 and OX18 immunoreactivities were seen in the nucleus (n.) gracilis, but not the n. cuneatus, at 2 days postlesion. At 5 days postlesion, OX42 immunoreactivity was markedly decreased, but class I MHC antigen expression was still enhanced. GFAP immunoreactivity increased only in the n. gracilis and remained elevated 2-12 days postlesion. Microglial activation is an early lesion-induced event in the CNS, and activated microglia may play a role in mediating the regenerative capacity of injured CNS axons.