Using single fibre vagal afferent recording techniques, we have investigated the longitudinal and circumferential location of slowly adapting stretch receptors in a functionally isolated, in situ segment of the trachea in dogs. 2. We have also studied the depth of these receptors within the airway wall and their response to reflex and drug induced contraction of the airway smooth muscle. 3. Thirty-four per cent of the receptors studied were in the extrathoracic trachea; calculations indicate that about 17-1% of all airway stretch receptors lie outside the thorax. 4. All the receptors were located in the membranous posterior wall of the trachea, and those tested responded more to transverse than to longitudinal stretching of the wall. 5. Receptors in the extrathoracic trachea continued to function after the regional mucosa had been widely resected; subsequent removal of histolocially proven smooth muscle tissue caused the abrupt cessation of receptor discharge. 6. Contraction of airway smooth muscle brought about an increase in receptor discharge frequency. 7. Similar results were found for a smaller number of stretch receptors located in the mainstem and lobar bronchi. 8. These findings procided evidence that stretch receptors are anatomically located within the smooth muscle layer of the airways and are functionally aranged in series with the muscle fibres.
