The nasal mucociliary apparatus is an important component of the airway defenses. Studies were undertaken to determine the nature and distribution of acute effects of inhaled formaldehyde on the nasal mucociliary apparatus of male F-344 rats using whole body exposures. Formaldehyde exposures ranged from a single 6-hr period up to multiple 6-hr exposures daily for 3 weeks, with exposure concentrations of 15, 6, 2, 0.5, and 0 ppm. Within 1 hr of the last exposure, the rats were killed and the nasal passages examined for effects on nasal mucociliary function. Exposure to 15 ppm formaldehyde induced inhibition of mucociliary function in specific regions of the nose, and mucostasis was generally more extensive than ciliastasis. These effects, which were initially confined to the anterior regions of the nose, became progressively more extensive for up to 2 weeks of exposure with only very slight progression during the third week. Inhibition of mucociliary function was much less severe with 6 ppm, minimal at 2 ppm, and not detected in rats following exposure to 0.5 ppm. The distribution of epithelial lesions, identified by histopathology, correlated well with the distribution of defective mucociliary function, but mucociliary function was a more sensitive indicator of toxicity. Localized defects in mucociliary function represent a potentially important consequence of exposure to formaldehyde.