An in vivo preparation was designed to investigate the effect of vagus nerve stimulation-induced bronchoconstriction on the relationship of slowly adapting pulmonary stretch receptor (SAR) activity and lung mechanics. SAR activities were recorded from the left vagus nerve. The responses of SARs, total lung resistance (RL), and dynamic lung compliance (Cdyn) to electrical stimulation of the peripheral end of the cut right vagus nerve (10-15 V, 5-30 Hz, 0.2 ms) were examined before atropine and 5 and 10 min after atropine (2 mg/kg) in anesthetized, artificially ventilated, bilaterally vagotomized rabbits. In the time course profile during vagal stimulation, an increase in RL and a decrease in Cdyn occurred simultaneously, and these opposite changes were frequency dependent. The average responses of SAR activity, RL, and Cdyn to vagal stimulation became more pronounced as the frequencies of the stimulation were increased. The responses obtained during vagal stimulation (5-30 Hz) were blocked or diminished greatly by the administration of atropine. Repeated vagus nerve stimulation in the presence of atropine did not show any significant change in SAR activity and lung mechanics. These results suggest that changes of SAR activity, RL, and Cdyn induced by vagal stimulation occur as a result of smooth muscle contraction in the airways, which is mediated mainly by muscarinic receptor activation and which is not involved in the release of neurotransmitters to relax airway smooth muscle.