The mammalian suprachiasmatic nucleus (SCN) is considered to be a critical component of a neural system implicated in the temporal organization of a wide variety of biological processes. Since the environmental light-dark cycle is the main zeitgeber for many of these rhythms, photic information may have a synchronizing effect on the endogenous clock in the SCN by inducing periodic changes in the activity of certain groups of neurons. The present study was conducted to investigate whether the daily light-dark cycle as well as seasonal variations in photoperiod would affect the vasopressin cell population of the human SCN. To that end, the brains of 30 young human subjects (ranging in age from 6 to 47 years) were investigated. We found that the subdivision of the human SCN that contains vasopressin-producing neurons fluctuated significantly over the 24-hr period. The volume of the vasopressin cell population was, on average, 1.4 times as large during the daytime (1000-1800 hr) as during the nighttime (2200-0600 hr), and contained 1.8 times as many vasopressin-immunoreactive neurons. Peak values in both vasopressin volume and vasopressin cell number were observed in the early morning (0600-1000 hr). In general, the SCN contained fewer vasopressin-immunoreactive neurons during the night than during any other period of the natural light-dark cycle. In addition to the diurnal cycle of the SCN, a marked seasonal rhythm was observed. The volume of the vasopressin cell population was, on average, 2.4 times as large in the autumn as in the summer, and contained 3 times as many vasopressin-immunoreactive neurons. In general, the annual cycle of the human SCN showed a nonsinusoidal pattern with a maximum in early autumn, a lower plateau in winter, and a deep trough in late spring and early summer. In contrast with the periodic fluctuations in the number of vasopressin-immunoreactive neurons in the SCN, no significant diurnal or seasonal variations could be detected in the numerical cell density or cell nuclear diameter of vasopressin neurons. In conclusion, the findings indicate that the synthesizing activity of the vasopressin neurons of the human SCN exhibits a diurnal as well as a seasonal rhythm, and that the temporal organization of these processes becomes disturbed later in life.