The melanocytes are acutely sensitive to a single pulse of UV and express neural differentiation. The present work was undertaken to observe whether the melanocyte can sense variations in the duration of UV exposure. Whole skin organ cultures from marginal zone in vitiligo were exposed to a single pulse of UV, 30, 60, 90 and 120 s each. Catecholoxidase levels in the marginal melanocytes and the volume of melanocytes were used to calculate and quantitate the changes in enzyme production. The melanocytes’ dendricity, volume and enzyme production increases with the duration of UV exposure. This sensitivity of the marginal melanocytes, to changes in the duration of UV exposure, simulates the coat color changes in weasels and the polar fox exposed to extreme variations in the day/night cycles. The UV response is associated with proliferation of melanocytes as it is G2-phase dependent. Thus the melanocytes form a UV-sensitive neural network responding to annual changes in the photoperiodicit