The morphological aspects and density of the serotonergic innervation in the forebrain of young (2 months) and aged (28-32 months) rats was studied employing immunocytochemistry with an antibody to serotonin. In the aged rats aberrant morphology of many of the preserved fibers was observed. The aberrant fibers were characterized by swollen varicosities and swollen intervaricose connections. They formed small networks. These findings were mainly restricted to the frontoparietal cortex and caudate putamen. In the same regions we observed a decrease in serotonergic innervation. There was no overall relation between aberrant morphology and decrease of serotonin-innervation as we observed a decrease in fiber density without morphological abnormalities in the hippocampus. It is suggested that the aberrant morphology may reflect the local degeneration of serotonergic forebrain afferents during the process of aging.