In open chest anaesthetized dogs the haemodynamic effects of solutions of equal hyperosmolarity (viz. NaHCO3 8%, NaCl k.6%, and glucose 34.3%, solutions) given into the bronchial artery were studied. Administration of any of these solutions directly into the bronchial artery resulted in increased cardiac output, stroke volume, bronchial blood flow, and bronchial fraction of the cardiac output, and decreased heart rate and bronchial as well as pulmonary vascular resistances. When given into the pulmonary circulation, the same solutions evoked similar reactions of smaller magnitude. To exclude the effect of major surgical trauma and the open-chest condition, another experimental model closer to the physiological situation was also developed. In this preparation NaHCO3 failed to produce the above haemodynamic response even when given into the bronchial artery. After a one-hour bleeding period resulting in a drop of arterial blood pressure to 40 mmHg, while using the same preparation, the administration f NaHCO3 solution into the bronchial artery caused a significant rise in blood pressure in both the systemic and pulmonary arteries. In these experiments a correlation was found between arterial oxygen tension and the extent of change in blood pressure. The exact mechanism of action of the observed haemodynamic changes is still not clear. However, it is likely that receptors localized in the area of the bronchial circulation and sensitive to hypoxia might have played a role in the development of the haemodynamic effects described.