The effect of a beta-adrenoceptor stimulating agent, terbutaline, on mucociliary transport in the lungs was studied in 10 patients with chronic bronchitis. Mucociliary transport was studied by having the patients inhale 6mum teflon particles tagged with 99mTc and by external measurement of the radioactivity in the lungs in the supine position. Terbutaline 0.25 mg and vehicle respectively were given subcutaneously in a cross-over double-blind study. On the average, clearance was slow in the examinations where the patients were given vehicle. Terbutaline produced a marked increase in mucociliary transport in four patients, a smaller increase in one and no effect in the others. Three out of the four patients who showed a marked inrease in clearance had less ventilatory impairment than the other seven patients. This may indicate that the mucociliary transport mechanism is less severely damaged in relatively early stages of the disease than in later stages. On the average FEV1.0 deteriorated significantly during the clearance measurements when vehicle was given, but did not change significantly when terbutaline was given.