In anaesthetised guinea-pigs, bronchoconstriction induced by vagal nerve stimulation was potentiated by low doses of the antimuscarinic bronchodilator drug, ipratropium (0.01-1.0 microgram/kg); the maximum effect was obtained with 1.0 microgram/kg which doubled the bronchoconstriction. When the dose was increased above 1.0 microgram/kg potentiation no longer occurred; instead the vagally induced bronchoconstriction was antagonised. This was accompanied by reduction in the bronchoconstriction and bradycardia induced by i.v. acetylcholine, due to blockade of post-junctional muscarinic receptors in the airways and heart. With 10 micrograms/kg ipratropium responses elicited both by vagal stimulation and by exogenous acetylcholine were abolished. The results show that ipratropium is an antagonist for pre-junctional muscarinic inhibitory receptors on pulmonary parasympathetic nerves and also confirm its potent antagonist actions on post-junctional muscarinic receptors in the airway smooth muscle. The effect of ipratropium in the lung depends, therefore, on the balance between the pre- and post-junctional effects.