Studies were conducted to measure the effect of graded concentrations of nickel on ciliary activity in an in vitro model system using isolated hamster tracheal rings, and to compare the in vitro findings with in vivo exposures using an organ culture system. Dose-response experiments were conducted for various Ni concentrations and for various lengths of exposure. In the in vitro model, significant decreases in ciliary activity occurred at concentrations as low as 0.011 mM nickel (0.65 microgram of nickel per ml). A dose-response relationship between the concentration of nickel and ciliary action was demonstrated. A range of effects from a significant decrease of 26 beats per min (0.65 microgram of nickel per ml) to complete ciliostasis (65 microgram of nickel per ml)) was observed after 24 hours of exposure. Validation of the in vitro results occurred when hamsters were exposed to a nickel chloride aerosol at concentrations of 100 to 275 microgram of nickel per m3. Immediately after the cessation of a 2-hour in vivo exposure, a decrease in ciliary activity of 60 and 140 beats per min was observed for animals exposed to 100 and 275 microgram of nickel per m3, respectively. The length of time required for recovery from the in vivo exposure to nickel aerosol was determined. Histologic examination of the epithelial cells also showed dose-related effects. It is apparent that nickel, either administered to the whole animal or to the tracheal epithelium directly, can significantly decrease the ciliary beating frequency and interfere with the normal functioning of respiratory defense mechanisms, thereby increasing the risk of the host to respiratory infections.