Large neurons in the nucleus tegmenti pedunculopontinus pars compacta (TPC) of the rat were shown to have choline acetyltransferase (ChAT) immunoreactivity by light and electron microscopic immunohistochemistry using a monoclonal antibody to ChAT. The ChAT-positive TPC neurons had triangular or multipolar cell bodies with diameters of 20-50 micron. The ultrastructural features of the ChAT-positive TPC neurons were similar to those of ChAT-positive neurons in the caudoputamen; each neuron had a distinct, pale nucleus with an indented nuclear envelope and large, cytoplasm-containing, well-developed endoplasmic reticulum and pale mitochondria.