Ground-based observations of stratospheric ClO, N2O, and HNO3 were made almost continuously at McMurdo Station, Antarctica (77.9 ¿S, 166.6 ¿E), during the austral spring of 1994, using two separate microwave receivers. Vertical profiles of these trace gases have been retrieved from the pressure broadened emission spectra between September 4 and October 8, 1994. In early September, McMurdo was located well inside the polar vortex, and high mixing ratios of chlorine monoxide (up to 1.8 ppbv) were measured in the lower stratosphere. Because of vortex movement, later measurements were taken in edge regions, where ClO was found to be quite variable. This vortex movement also provided an opportunity to study relative changes between all three species. Almost no HNO3 was seen below 20 km during the measurement period, indicating that stratospheric air had been efficiently denitrified by polar stratospheric cloud formation. A significant increase of the nitric acid column was observed only around September 20, when McMurdo was closer to the outer edge of the vortex. At the beginning of the measurements, the vertical profiles of the inert tracer N2O had already descended so far that very little N2O was present above 20 km. During the observation period, the N2O distribution did not show strong changes except for a slight downward trend which increased with altitude. This indicates, as noted in previous years, that subsidence continued in the stratosphere over McMurdo Station until at least early October, when measurements were stopped. The temporal correlations between the behavior of ClO, N2O, HNO3, altitude, and temperature at the 50-hPa level, and of ozone measured by local ozonesondes show that changes in the atmospheric composition were partly due to dynamic effects. A backward trajectory analysis was performed to interpret the ClO data in an attempt to clarify some irregularities. ¿ American Geophysical Union 1996