The mammalian suprachiasmatic nucleus (SCN) is considered to be the principal component of the biological clock, generating a wide variety of physiological and behavioral circadian rhythms. In addition to its implication in circadian rhythms, the SCN is also involved in the expression of some seasonal cycles. In the present study we show that the volume of the human SCN and the number of vasopressin-immunoreactive neurons contained within it fluctuate significantly over the year, with values being about twice as high in the autumn as in the summer. In contrast, no seasonal variations could be detected in the paraventricular nucleus (PVN), another major vasopressinergic cell group in the hypothalamus. The present findings suggest that the human SCN is influenced by seasonal variations in photoperiod.