AbstractIn immunocytochemical studies, a polyclonal antiserotonin antibody was used to visualize fibers within the cingulate cortex of young and aged rhesus monkeys. Intricate and distinct patterns of serotoninergic processes were seen in anterior and posterior segments of cingulate cortex (Brodmann areas 24 and 23). In these regions of cortex, many multivaricose serotonin‐immunoreactive axonal swellings were identified, and some of these immunostained neurites were associated with deposits of amyloid. These observations suggest that serotoninergic processes are involved in the formation of senile plaques in neocortex of aged macaque