Eight subjects with asthma underwent bronchial challenge with histamine and methacholine. Dose-response curves were drawn on a scale which made the dosage equivalent in molecular weight. The results were analysed in terms of both the slope of the dose-response curve and the dose required to elicit a 20% fall in FEV1. No significant difference between methacholine and histamine was found in either measurement. Because of the similarity of the responses we conclude that the two agents are similar in action and may be used with equal effectiveness in bronchial challenges.