The effect of age on the number of neurons in the nucleus centralis superior (NCS) was determined in 11 behaviorally tested rhesus monkeys of 7-32 years of age, There was a significant age-related decrease in both cell packing density and in the total number of neurons. This decrease in number of neurons appeared to effect two different populations of cell in the NCS, one of which corresponded in size to the serotonergic cells in this nucleus and the other to a smaller-sized cell. Comparisons of the changes in the cell packing density with behavioral testing, showed significant correlations with the overall test performance as well as with individual tests of memory function and of executive system functions. These findings suggest that neuronal loss in the NCS may play a significant role in mediating cognitive changes seen in normal aging.