A 25-year-old man with solar urticaria is described. The action spectrum ranged from 400 to 500 nm. An inhibition spectrum was found to be in the visible light range above 660 nm. Simultaneous or alternate exposure to 'blue-violet light' and 'red light' mostly inhibited weal formation. The urticarial reaction was not blocked by local injection of antihistamines and not prevented by histamine depletion with polymyxin B sulfate. These data suggest that histamine may not play a major role in weal production in this case.