Loratadine is a new non-sedating antihistamine. The present studies compared loratadine and terfenadine, another non-sedating antihistamine, for their ability to inhibit the bronchial response to histamine and other autacoids which have been implicated as contributing to the symptoms of an allergic reaction. In addition, the two antihistamines were evaluated in models of immunologically mediated allergic reactions. Loratadine is a more potent inhibitor of histamine-induced bronchospasm in guinea pigs than is terfenadine. Both antihistamines exhibit marked antiserotonin activity at doses 10 times their antihistamine ED50 values. In contrast, loratadine and terfenadine produce little or no inhibition of the bronchial responses to methacholine, leukotriene C4 or platelet-activating factor. An allergic bronchospasm in guinea pigs is inhibited by loratadine (ED50 = 0.40 mg/kg, p.o.) and terfenadine (ED50 = 1.7 mg/kg, p.o.). The bronchospasm associated with allergic anaphylaxis in rats is significantly inhibited by 10 mg/kg, p.o. loratadine and 30 mg/kg, p.o. terfenadine. Loratadine exhibits antiallergy activity in vitro. At micromolar concentrations, loratadine inhibits the release of histamine from Con A and A23187-stimulated rat peritoneal mast cells and the release of histamine and leukotriene C4 from a Con A-stimulated cloned murine mast cell line.