Experiments on dogs have shown that histamine inhalation in small dosages increases the sensitivity of the acetylcholine receptors of the bronchial muscles. Increased reactions following acetylcholine inhalations up to 11 times the control values were measured. After the change of sensitivity, antihistamine cannot influence the increased reactivity of the bronchial muscles. Antihistamine increasesthe histamine concentration of the blood. This increase was not seen in experiments where antihistamine was given after the inhalation of histamine.