The number of neurons expressing vasopressin (AVP) was determined in the suprachiasmatic nucleus (SCN) of the hypothalamus of 41 human subjects ranging in age from 27 weeks of gestation up to 30 years of age. At term, i.e. at 38-42 weeks of gestation, the fetal SCN is still immature, since it then contains only about 13% of the adult number of AVP-expressing neurons. The period of rapid increase in the number of AVP-staining neurons in the SCN during the first few months postnatally coincides with the appearance of overt diurnal rhythms in the child.