Distension of the main pulmonary artery and its bifurcation are known to result in a reflex vasoconstriction and increased respiratory drive; however, these responses are observed at abnormally high distending pressures. In this study we recorded afferent activity from pulmonary arterial baroreceptors to investigate their stimulus-response characteristics and to determine whether they are influenced by physiological changes in intrathoracic pressure. In chloralose-anaesthetized dogs, a cardiopulmonary bypass was established, the pulmonary trunk and its main branches were vascularly isolated and perfused with venous blood at pulstatile pressures designed to simulate the normal pulmonary arterial pressure waveform. Afferent slips of a cervical vagus were dissected and nerve fibres identified that displayed discharge patterns with characteristics expected from pulmonary arterial baroreceptors. Recordings were obtained with (a) chest open, (b) chest closed and re-sealed, and (c) with phasic negative intrathoracic pressures in the resealed chest. Pressure-discharge characteristics obtained in the open chest animals indicated that the threshold pulmonary pressure (corresponding to 5 % of the overall response) was 17.1 ± 2.9 and the inflexion point of the curve was 29.2 ± 3.3 mmHg (mean ± sem). In closed chest animals the values for the threshold and inflexion pressures were 12.0 ± 1.7 and 20.7 ± 1.8 mmHg. Application of phasic negative intrathoracic pressures further reduced the threshold and inflexion pressures to 9.5 ± 1.2 mmHg (P < 0.05 v's open) and 14.7 ± 0.8 mmHg (P < 0.003 v's open; P < 0.02 v's atmospheric). These results indicate that under physiological conditions, with closed chest and phasic negative intrathoracic pressure changes similar to those associated with normal breathing, activity from pulmonary baroreceptors is obtained at physiological pulmonary arterial pressures in intact animals.