Histamine release in cold urticaria was studied before and after therapy with cyproheptadine to determine whether any effect upon histamine release could be distinguished from end-organ receptor-site blockade. Patients were asymptomatic while taking cyproheptadine, their ice cube test reverted to normal, and only one of six patients had any swelling upon ice-water submersion of one hand for 5 min. Histamine-release curves were obtained following ice-water submersion before and after cyproheptadine therapy. All patients had significant histamine release and in five of six patients there was no evident difference in the magnitude of histamine release before or after therapy. A single patient did have diminished histamine release after therapy, but could not be restudied to be sure this was not a spurious result. Our data demonstrated that cyproheptadine is extremely effective in ameliorating the symptoms and signs of cold urticaria and that its principal effect is that of an H1 receptor antagonist, thereby blocking the effects of histamine. These data further suggest that a sufficient dose of any standard antihistamine should be similarly effective and that patients who do not tolerate cyproheptadine or do not appear to respond to it should be tried on other antihistamines of the H1 type.