A series of proton and electron injections were observed by Explorer 45(S3) associated with several substorms during the main phase of the February 24, 1972, geomagnetic storm. The 1- to 290-keV protons and 1- to 560-keV electrons were observed in the evening quadrant up to L~5.2. A model distorted dipole magnetic field and McIlwain's E3 convection electric field were used to backtrack the energy-dispersed electron and proton fluxes to their source at the time of injection. The source turns out to be a region extending over several earth radii outside an injection boundary. For the pesent storm, in the night magnetosphere, the inferred injection boundary is displaced inward with each successive substorm. The energy dispersion plot of the particles is injected during orbit 314 indicates that as the energy of the observed particles decreases there is a smooth transition to the position of the plasmapause. This suggests that for that substorm the injection boundary and the plasmapause were one and the same. Within the framework of the model used, the proton 'noses' reported by Smith and Hoffman [1974> can be attributed to a combination of the shape of the injection boundary and the drift dynamics of injected protons and electrons.