In mammals, all epidermal melanocytes react to exposure by ultraviolet light by increased activity and population. The experiments reported here were designed to reconfirm Yun and Montagna's findings and to determine one the long-term effects of UV light on pigment cells 2 whether stimulated. Pigment cells can be restimulated after a period of rest and three, whether tyrosinase activity increases and remains at a high level with UV stimulation or decreases once it reaches this peak, and for how ultra structural features relate to DOPA - positivity in melanocytes.