Antihistamines are effective therapy for histamine-mediated conditions, including seasonal and perennial allergic rhinitis and chronic urticaria. Until recently, all antihistamines produced some degree of drowsiness, as well as anticholinergic side effects. Several nonsedating antihistamines have been developed. Two of these antihistamines--terfenadine and astemizole--are commercially available. The lack of central nervous system effects is attributed to the inability of these drugs to penetrate the blood-brain barrier. They also have no appreciable binding to cholinergic receptors. Clinical trials have demonstrated that the newer agents are as effective as classic antihistamines and that they have no greater incidence of central nervous system or anticholinergic side effects than placebo.