Choline acetyltransferase (ChAT) was localized immunohistochemically within the brain of the guinea pig using a monoclonal antibody. ChAT was found in the cytoplasm of cell bodies and primary dendrites of neurons located in striatum, basal forebrain, cranial nerve motor nuclei and scattered cells in the pons. The greatest numbers of immunoreactive neurons were located in the diagonal band of Broca, medial septum and striatum. Distinct immunoreactive fibers were not visible using this antibody, although a diffuse immunostaining was present in the same nuclear regions as well as in the nerve roots of cranial nerve nuclei and the interpeduncular nuclei. Results of the present study agree closely with other previous reports of acetylcholine distributions.