Pharmacological studies have suggested that the m2 muscarinic receptor functions as an autoreceptor in the cholinergic axons which innervate the cerebral cortex and striatum. To test this hypothesis in the macaque monkey, we used a subtype-specific antibody to the m2 muscarinic receptor. Immunoreactive cells were well visualized in the nucleus basalis, where some of these cells displayed dense m2 immunoreactivity, while others were lightly labeled. This heterogeneity of labeling intensity was not based on peculiarities of the methodology, because cholinergic cells of the striatum expressed uniformly dense m2 immunoreactivity. Concurrent labeling with choline acetyltransferase immunoreactivity proved that most of the heavily m2-labeled cells in the nucleus basalis were also choline acetyl-transferase positive. The findings demonstrate that at least 10-25% of the cholinergic neurons in the nucleus basalis of the monkey are densely m2 immunoreactive. In the striatum, concurrent labeling demonstrated that the majority, if not all, choline acetyltransferase-positive cells also contained m2 immunoreactivity. In addition, these experiments identified a population of smaller striatal cells which were m2 immunoreactive and choline acetyltransferase negative. Consecutive labeling with m2 immunoreactivity and NADPH-diaphorase histochemistry demonstrated that many of these m2-immunoreactive non-cholinergic neurons belonged to the population of nitric oxide-synthesizing medium aspiny neurons. The findings indicate that the m2 muscarinic receptor may be expressed at high levels in only a subset of cholinergic basal forebrain neurons. In contrast, m2 receptors appear to be expressed by all cholinergic cells of the striatum.