1. Ventilation-perfusion balance in the presence of airway obstruction will depend on the efficiency of hypoxic pulmonary vasoconstriction beyond obstructed airways and the matching of redistributed blood flow and ventilation to the rest of the lung. This study investigated the relative importance of these mechanisms in man during experimental bronchial occlusion. 2. The bronchus to the left lower lobe was temporarily occluded with a balloon-tipped catheter during fibreoptic bronchoscopy in eight supine normal volunteers. Respiratory gas tensions were measured within the occluded lobe with a respiratory mass spectrometer. The distribution of ventilation and perfusion was assessed under control conditions and after 5 min of bronchial occlusion by computer analysis of the regional distribution of radioactivity during inhalation of 81mKr gas and following injection of 99mTc-labelled macroaggregated albumin respectively. 3. Respiratory gas partial pressures within the occluded lobes rapidly stabilized at mixed venous gas tensions: PO2 43.4 +/- 2.2 (SEM) mmHg, PCO2 40.2 +/- 1.8 mmHg. During occlusions the arterial oxygen saturation fell from a baseline of 96.3 +/- 0.46% to a nadir of 92.1 +/- 0.43%. Bronchial occlusion produced underventilation in the left lung relative to perfusion, both in the region of the occluded lower lobe and at the lung apex. Relative overventilation occurred in the right lung. 4. It is concluded that arterial hypoxaemia during lobal bronchial occlusion is caused primarily by shunting of mixed venous blood, though the shunt fraction is reduced by approximately 50% by hypoxic pulmonary vasoconstriction. In lung adjacent to obstructed regions reduced compliance may impair ventilation more than perfusion to contribute to hypoxaemia.(ABSTRACT TRUNCATED AT 250 WORDS)