Respiratory resistance was measured by a forced oscillation technique in vagotomized guinea-pigs before and after pulmonary micro-embolism produced by I.V. injection of 0.5 ml./kg of 10% w/v BaSO4. Changes in quasi-static inspiratory compliance and arterial platelet count were also measured. 2. Micro-embolic challenge with BaSO4 increased respiratory resistance by 27%; this was abolished by prior treatment with indomethacin; no change in the control resistance occurred after indomethacin. 3. No change in respiratory compliance or arterial platelet numbers were observed following low dose BaSO4 micro-embolism. This suggests that pulmonary micro-embolism produced a decrease in medium or large airway calibre, which was mediated by a prostaglandin-like substance from lung tissue, and did not require the presence of the vagus nerves.
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