Light physiologically regulates the levels of c-Fos in the suprachiasmatic nucleus (SCN), but the identity of most of the cells that express this DNA-binding protein is unknown. To determine whether SCN c-Fos expression is restricted to neurons, we performed light microscopic double-label immunohistochemistry on sections of rat SCN using two c-Fos antibodies in conjunction with antibodies to neurofilament and S-100 proteins (as markers of neuronal and astrocytic cell bodies, respectively). In addition to c-Fos labeled nuclei in neurons, we found a small population of labeled nuclei in glial cell bodies. These observations suggest that c-Fos is expressed in a heterogeneous cell population in the SCN and that its function in circadian clock mechanisms may be cell specific.