The pars cerebellaris loci coerulei is a flat accumulation of perikarya representing a subnucleus of the locus coeruleus in the roof of the fourth ventricle of the brain in man. A quantitative study was done on this subnucleus; it comprised 67 specimens from individuals between 11 and 90 years. Nerve cell counts were performed on cleared mounted block preparations. Mathematical analysis of the data was performed using logistic growth functions. The results showed that with advancing age there is a loss of neurons in both sexes. This begins in the female approximately after age 40; in the male already at age 20 (reaching a lower value in old age). The final loss of neurons is more pronounced in the male than in the female. Male and female specimens did not differ in the total number of neurons. In both sexes the left side of the pars cerebellaris loci coerulei contained more neurons than the right.