We recently described a radioactive tracer method of measuring mucociliary transport rates (MCTR) in vivo in dogs. The distance from larynx to carina is measured on inserting a calibrated fiberoptic bronchoscope into the trachea. A small volume (0.04 to 0.1 ml) of a suspension of radio-labeled albumin microspheres 5 to 7 micrometer in diameter is then deposited on the mucosal surface at the carina via a catheter placed through the inner channel of the bronchoscope. The rate of movement of these sphere towards the larynx is visualized sequentially by means of a gamma scintillation camera with a large field of view and is recorded as polaroid images during 10 to 30 min. By using this technique, transport rates were determined in the tracheas of 10 anesthetized dogs (17 experiments) before and 10 min after intravenous infusion of 0.4 mg of atropine sulfate; each animal served as its own control subject. Baseline MCTR values were determined by using microspheres labeled with 140-KeV technetium-99m. After infusion of atropine, MCTR values were again determined using micropheres labeled with 390-KeV indium-113m. Because of their widely differing energy spectrums, both test agents could be used in rapid succession by adjusting the spectrometer of the scintillation camera. MCTR increased in 13 experiments, did not change in two, and decreased in two. The percentage increase in MCTR after infusion of atropine was significant as compared to baseline values (30.0 +/- 0.6 per cent, mean +/- SE; P less than 0.05 by paired t test), as was the absolute change in MCTR after infusion of atropine (P less than 0.05 by 2-way analysis of variance). Therefore, the effect of atropine in improving mucociliary clearance was quantitatively similar to that previously reported for beta-adrenergic agonists.