The suprachiasmatic nucleus (SCN) is considered to be the endogenous clock of the brain, essential for the ovulation cycle and the temporal organization of sleep-wake patterns, among other things. Immunocytochemical staining with anti-vasopressin as a marker permitted a morphometric study of this nucleus in the human brain, which revealed that the shape of the SCN is sexually dimorphic. The shape of the SCN was elongated in women and more spherical in men. In both sexes a decrease in SCN volume and cell number was observed in senescence (80–100 years). The latter change was especially pronounced in patients with senile dementia of the Alzheimer type (SDAT). This suggests the presence of a structural defect in the SCN which underlies the general disturbance of biological rhythms in senescence and SDAT.