Controversies exist over the influence of inhalation anaesthesia on blood flow regulation in response to local alveolar hypoxia. This study investigates the blood flow diversion from a hypoxic to an oxygenated lung in anaesthetized and ventilated animals in comparison to unanaesthetized animals. Chronically instrumented adult ewes (n = 14, 32-45 kg) were intubated one week after surgery with a modified Carlen's tube, allowing separate ventilation of the left and right lung. In the awake state (n = 7) or after one hour of constant anaesthetic conditions (n = 7, halothane 1.6% and 2.4%), cardiac output (thermodilution) and left pulmonary blood flow (ultrasonic transit time) were evaluated. Then, under identical ventilatory conditions, the left or right lung, alternately, was rendered hypoxic for 10 min by ventilation with nitrogen. The contralateral lung was ventilated with oxygen. After 10 min, haemodynamics were again recorded. The changes in left pulmonary blood flow under unilateral lung hypoxia were similar either in the awake or the anaesthetized state. Thus, we conclude that, under these experimental conditions, halothane anaesthesia and mechanical ventilation have no influence on blood flow regulation under unilateral lung hypoxia.