Urticaria induced by physical agents is an infrequent phenomenon but one that has been widely studied. Heat, cold, sunshine, trauma and probably other physical agents are capable of producing it. In persons in whom urticaria is produced by heat the attacks can also be precipitated by exercise and emotional stress. In fact, there is an interaction of these three factors which makes difficult the evaluation of any one of them. Of persons in whom attacks are produced by heat, effort and emotion, there are two groups: those who have urticaria and those who have generalized itching without urticarial lesions. The urticarial group has had the most attention; the second has been neglected, though the fundamental mechanism in each is probably similar. There has been a tendency to try to explain the reaction in persons with all types of physical urticaria on the basis of release of