Adenosine triphosphate (ATP) has been shown to revitalize the disturbed nasal mucociliary function in man. We investigated the effects of ATP on the ciliary beat frequency (CBF) in animals by immersing tracheal explants from rats in various concentrations of ATP, and by infusing ATP intravenously to guinea pigs. CBF was measured with a photodetector technique from the surface of the explants or from the incised trachea. ATP (from 0.01 to 1 mg/ml) in vitro increased CBF in rat tracheal explants up to 10.5% (p less than 0.05). In vivo ATP (1 mg/kg) increased the CBF by 29% (p less than 0.01) in the guinea pig trachea. As the CBF was increased by ATP, both in vitro and in vivo, it can be suggested that the improvement in mucociliary transport by exogenous ATP as shown in previous studies is caused by the ciliostimulatory effect of ATP.