Velocity distribution functions of both the electron and the proton component of the magnetospheric plasma are measured in 62 logarithmically spaced steps from 50 eV to 50 keV by an instrument carried upon the ATS‐5 spacecraft in circular synchronous orbit (6.6 earth radii). Oscillations which appear to be collisionless drift waves having periods of several minutes or longer are found in this data. Linearized wave theory can be use to predict the theoretical perturbation distribution functionfTin terms of the perturbing fields and the equilibrium distribution functionF. The functional dependence of the theoretical perturbation distribution functionfTupon particle energy can then be compared with that of the measured perturbationfM. The linear wave theory is found to predict correctly a change in the sign offMwhich occurs at a critical particle energyE*and the relative magnitude offMover the entire energy range.