The behavior of particles injected during three well-defined substorm events that occurred on August 18, 1974, has been studied in detail. Plasma characteristics from ∼200 eV to 80 keV detected at the ATS 6 position have been studied in detail, and we suggest that the results are consistent with the idea that there are two particle sources for electrons during substorms: one at small pitch angles, predominantly in the magnetic field direction, and the other at large pitch angles. The sources of these electrons are probably the ionosphere and the plasma sheet. For protons the results suggest a one-particle source. One possible source for the protons is the plasma sheet. The results have been interpreted in a framework of a model involving magnetospheric convection and parallel electric field.