||N-acetylcysteine (NAC) is widely used as a mucolytic agent, but the clinical and pharmacological effects of NAC are still unclear. It has recently been claimed in animal studies that NAC will stimulate ciliary beating frequency at low concentrations, while inhibiting beating at higher concentrations. Using a microphoto-oscillographic method combined with microperfusion technique, we studied the direct effect of NAC on human nasal cilia. NAC caused a direct dose- and time-related decrease in ciliary beating frequency, which was detectable at 2 mg/ml and reached statistically significant levels at 20 mg/ml. NAC at 200 mg/ml caused total cessation of movements within 15 s but this effect was completely reversible within 15 min of perfusion with medium alone. At no concentrations of NAC was beating frequency increased over base-line. NAC, however, had no effect on beating pattern. NAC seems to have a selective and fully reversible inhibitory effect on the beating frequency of human cilia in vitro.