Mucociliary transport and the percentage of ciliated cells were studied in large and intermediate sized airways (generations 0-3) of anesthetized mongrel dogs before and after aminophyllin administration. The mean percentage of ciliated cells decreased respectively from 22.0% and 25.4% in the trachea and main bronchi to 8.0% in the lobar and 2.7% in the segmental bronchi. The fastest mean mucous velocity was found in the trachea (4.9 mm/min, S.E. 1.0). Mucous velocity decreased 38% from the trachea to the main bronchi, 55% to the lobar bronchi, and 66% to the segmental bronchi. A positive correlation (r = 0.75) was demonstrated between mucous velocity and the percentage of ciliated cells. After aminophyllin administration, mucous velocity increased 59% in the trachea and 27% in the main bronchi. No statistical difference in mucous velocity could be demonstrated in the smaller airways after aminophyllin, probably due to relative insensitivity of the method in short bronchi with low mucous velocities.