The bronchodilator effects of ketamine were examined in human bronchial preparations contracted maximally with histamine, acetylcholine, barium chloride or potassium chloride. Antagonism between ketamine and either histamine or acetylcholine was examined also. Ketamine caused bronchial relaxation irrespective of the constricting agent, and exerted a partial and non-competitive antagonism to histamine and acetylcholine. Propranolol and indomethacin did not inhibit the effect of ketamine, excluding the involvement of beta activation and of prostaglandins.