Acute effect of a short-term exposure to cigarette smoke on mucociliary clearance mechanism was studied in 69 anesthetized dogs. A tracer material, ^99mTc-MAA (macroaggregated albumin), was placed in the right main bronchus near the carina 2 min after completion of exposure and its migration distance on the tracheal mucosa with time was assessed by a scintil-lation camera, and the mean migrating velocity (MMV) was calculated. The average MMV values were 12.0±1.0 (mean±S. E. ) mm/min in the control dogs, while the values were 11.6±1.5, 8.3±2.2, 3.3±0.8, and 4.2±2.3 mm/min in the dogs which smoked 1, 3, 5 and 8 cigarettes, indicating that there was an inverse correlationship between the number of cigarettes smoked and the MMV values. When cigarette smoke was filtered through a glass fiber filter, the average MMV values were 7.0±0.5 and 1.3±0.9 mm/min in the dogs which smoked 5 and 8 cigarettes respectively, suggesting that once the number of cigarettes amounted to eight, even cigarette smoke filtered through a glass fiber filter was as detrimental to the mucociliary clearance mechanism as the non-filtered cigarette smoke.