Observations of substorm-associated enhancements of proton and electron fluxes were made by Explorer 45 below L=5.3 in the dusk magnetosphere on February 13, 1972. The particles were observed ater a substorm that started at 1116¿0010 UT in the wake of an ssc at 0939 and that initiated a small magnetic storm with maximum Dst=47 &ggr;. The protons in the range between 1 and 40 keV exhibit strong dispersion effects in both energy and pitch angle. The electrons also show dispersion effects above 40 keV, while those below that energy are not observed until the satellite crosses the plasmapause. The observations are interpreted in terms of McIlwain's model electric field E3 and an injection boundary close to or coinciding with the plasmapause. Good quantitative agreement is obtained between the observed and the predicted proton energies and pitch angle dispersions and electron energy dispersions. In view of strong temporal fluctuations in the electron flux, no comparison in pitch angle dispersion of electrons is possible. The conclusion drawn is that in this event protons and electons were injected at the onset of the substorm more or less instantaneously into the magnetosphere along or above the injection boundary and particles with energies below 10 keV were not convected in from the magnetospheric tail.