Chest physiotherapy, including posturing the patient head downwards while the chest is percussed and vibrated, was used in the treatment of patients with an exacerbation of chronic bronchitis. In seven patients the mean FEV declined from 1.38 litres plus or minus 0.39 to 1.25 litres plus or minus 0.37 after postural drainage and percussion (P less than 0.001). Twenty minutes later the FEV rose to 1.37 litres plus or minus 0.33. The mean decline in the FEV1 was prevented by prior administration of salbutamol. The fall in the FEV1 did not occur in 10 patients who received the postural tipping without chest percussion. Also it was not induced by coughing every two minutes during posturing. It was considered that the fall in FEV1 after chest physiotherapy was due to bronchoconstriction caused by the chest percussion or vibration, particularly in patients with bronchoconstriction under basal conditions. The induced bronchoconstriction counter-balanced any improvement of the FEV due to freeing the airways of sputum, but in two patients with moderate to copious sputum an improvement of the FEV1 was repeatedly obtained in measurements made 20 minutes after the physiotherapy when the bronchoconstriction had presumably subsided. Although the immediate decline in FEV1 was not large, it is considered inadvisable to employ chest percussion and vibration in sick patients unless a bronchodilator is administered previously.