A test particle code is used to study the response of ensembles of particles to a two-dimensional, time-dependent model of the geomagnetic tail and to test the proposition (Coroniti, 1981a, b; B¿chner and Zelenyi, 1986; Chen and Plamadesso, 1986; Martin, 1986) that the stochasticity of the particle orbits in these fields an important part of the physical mechanism for magnetospheric substorms. The realistic results obtained for the fluid moments of the particle distribution with this simple model, and their insensitivity to initial conditions, are consistent with this hypothesis. ¿ American Geophysical Union 1990