The effects of 30 min exposure to sulfur dioxide on mucociliary activity (MCA) and ciliary beat frequency (CBF) were studied in 31 guinea pig tracheas. MCA was measured by recording the light reflected from ciliated mucous membranes using an infrared bar code reader CBF of single ciliated cells obtained by brushing was measured with phase-contrast microscopy Each tracheal sample was exposed to SO 2 at concentrations ranging from 2 5 to 12 5 ppm, or to air for control purposes. MCA and CBF were measured before and immediately after gas exposure A reduction in mean MCA of 63 % (P = 0 0007) and statistically insignificant changes in CBF (P > 0 05) were recorded at concentrations of 2 5 ppm SO 2 Higher SO 2 concentrations caused a further impairment of MCA as well as a dose-dependent decrease in CBF (P = 0 002) A concentration of 12 5 ppm SO 2 induced a decrease from baseline values of approximately 80 % in mean MCA and of roughly 70 % in mean CBF. This study demonstrates a dose-dependent SO 2 -induced decrease in MCA of guinea pig tracheas The decrease in MCA was associated with an impairment of CBF only at SO 2 concentrations higher than 5 0 ppm.