1. We studied the characteristics of pulmonary sensory receptors whose afferent fibres are in the left vagus nerve of opened-chest rats. The activity of these receptors was recorded during mechanical ventilation approximating eupnoea, as well as during deflation, stepwise inflations and constant-pressure inflations of the lungs. Data were also collected from closed-chest rats and analysed separately. 2. Ninety-four per cent of receptors were located in the ipsilateral lung or airways with the remainder in the contralateral lung. 3. Not only were slowly adapting receptors (SARs) the most abundant pulmonary receptors but 21% of them were either exclusively or predominantly active during the deflationary phase of the ventilatory cycle. Deflationary units were found in opened- and closed-chest rats. The average conduction velocity for all fibres innervating SARs averaged 29.7 m s-1. 4. We found rapidly adapting receptors (RARs) to be extremely rare in the rat. Their activity was sparse and irregular. The conduction velocities of fibres innervating RARs averaged 12.3 m s-1. 5. Far more abundant than RARs in the remaining population of pulmonary fibres were C fibres. They were observed to have an average conduction velocity of 2.1 m s-1, base-level activity which was irregular and a high pressure threshold of activation and were stimulated by intravenous capsaicin injection. 6. Notable differences exist between pulmonary receptors in rats and those reported in other species. The variations include the abundant existence of intrapulmonary SARs with exclusively deflationary modulation and the rarity of RARs. We also encountered C fibres which have not previously been described systematically in the rat.