Cell bodies in the rat suprachiasmatic nucleus (SCN) were labeled with an antibody against a small domain of theperiod(per) protein, the product of a gene inDrosophilathat regulates circadian rhythms. In immunoblots of SCN protein extracts, the antibody recognized a band of approximately 115 kD, as well as a heterogeneous antigen ranging from 160 kD to 170 kD. The antibody was found in previous studies to label putative circadian pacemaker neurons inAplysiaandBulla, as well as the cellular sites ofperexpression in flies. Taken together, these results suggest that the region of theperprotein recognized by this antibody may be widely conserved in neuronal circadian pacemakers.