The three-dimensional mass motions in a mechanistic numerical model of a sudden stratospheric warming are investigated by tracing a large number of marked particles. The model is adapted from that described by Holton (1976). A sudden stratospheric warming is simulated by allowing a large amplitude, single harmonic wave forced at the lower boundary of the model to interact with the zonal mean flow. The results of a model simulation involving zonal wavenumber 2 are reported. Rings of marked particles are placed along latitude circles at the time when the forcing is turned on at the lower boundary and their motions are traced for a period of 30 days. The particle displacements near the level of maximum warming are described in detail, the trajectories of particles exhibit two different types of characteristics depending upon whether the particles originate from high or low latitudes. The particle motions are therefore described in terms of two regimes, the polar and the tropical regimes. The polar regime is characterized by organized motions of the particles around the elongated polar cyclonic vortex. When projected onto the meridional plane, the rings of particles appear as ellipses whose centers of mass remain at the same location during the prewarming stage, but descend during the warming stage despite the fact that the zonal mean vertical velocities are directed upward in the same region at all times. The tropical regime is characterized by the trapping of some particles inside the cutoff anticyclones which first appear in middle latitudes and then migrate poleward. As a result, these particles spiral poleward and downward. Eventually the particles which originate from tropical latitudes are scattered all over the hemisphere. When projected onto the meridional plane, a cloud of particles spreads out poleward and downward until the particles are dispersed between the equator and the pole. These results of particle motions are interpreted in terms of the two quasi-conservative tracers, namely, potential temperature and potential vorticity. For conservative motions a thin tube bounded by two surfaces of constant potential temperature and two surfaces of constant potential vorticity always coincides with the same material line of air particles. These tubes bulge downward (upward) in the polar (tropical) stratosphere during the event of sudden stratospheric warming. It is shown that due to the Newtonian cooling processes the particles are displaced further downward (upward) away from the tubes from which they originate. The horizontal projections of these tubes are displayed and it is found that their shapes are much simpler than those of the material lines of air particles. The nonconservative processes which are responsible for these differences are discussed.