The intrinsic beating frequency of human ciliated respiratory epithelium at 3 different levels of the respiratory tree was examined. Ciliated epithelium was collected from the trachea, mainstem, and basal segment of 30 patients during fiberoptic bronchoscopy, and ciliary beat frequency was measured in vitro using a photoelectric technique. Specimens from the first group of 15 patients were measured at room temperature (23 to 25 degrees C) and those of the second group of 15 were measured at 37 degrees C. For group I the over-all mean beat frequencies at the trachea, right mainstem, and basal segment were 11.3, 11.1, and 11.4 beats/s, respectively, and for group II the over-all mean beat frequencies were 14.7, 14.7, and 14.8 beats/s, respectively. Within each group there were no significant differences (P greater than 0.05) in the over-all means recorded at the 3 sample sites. Although for a given subject, ciliary beat frequency was not significantly different for specimens taken from the trachea, mainstem, and basal segment, the pool mean beat frequency varied from 1 patient to another. Variations in beat frequency among patients could not be related to age, smoking habits, alcohol consumption, or disease history.