The mammalian suprachiasmatic nucleus is considered to be the major component of the biological clock, involved in the temporal organization of a wide variety of physiological and behavioral processes. The present study was conducted to investigate whether there are diurnal or annual variations in the morphology of the vasopressin-containing neuron population of the suprachiasmatic nucleus in human beings. To that end, the brains of 48 human subjects were investigated. A marked annual variation was observed in the volume and vasopressin cell number of the human suprachiasmatic nucleus: the volume of the vasopressin cell population was, on average, 2.5 times larger in October-November than in May-June and contained 2.7 times as many vasopressin-immunoreactive neurons. In general, the annual cycle of the human suprachiasmatic nucleus showed a non-sinusoidal pattern with a maximum in early autumn, a lower plateau in winter and a deep trough in late spring and early summer. In contrast, no such seasonal variations could be detected in suprachiasmatic nucleus vasopressin numerical cell density or cell-nuclear diameter. The number of vasopressin-immunoreactive neurons in the paraventricular nucleus of the hypothalamus, on the other hand, did not show any significant periodic changes over the year, indicating the specificity of the suprachiasmatic nucleus rhythm. In contrast with the annual cycle of the suprachiasmatic nucleus, no significant diurnal variations were observed in any of these parameters. In conclusion, the findings indicate that photoperiod may be considered a potential environmental factor controlling the activity of the vasopressin system of the human suprachiasmatic nucleus.