The UK Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model has been used to study the chemical evolution of the northern hemisphere polar vortex. A run that includes a parametrization of the heterogeneous reactions occurring on polar stratospheric clouds (PSCs) has been initialized with the ECMWF analyses of the 5th of January 1992. This run produces high values of active chlorine within the polar vortex, for example mixing ratios of ClOx (=Cl+ClO+2¿Cl2O2) of 1.8 ppbv are calculated after six days of integration, with correspondingly low values of HCl and ClONO2. During warm periods of this model run, when the PSC reactions are not active, the ClOx is converted slowly back into ClONO2. Chemical loss of ozone in January in the model is small.