Simultaneous observations of substorm-associated electron flux variations made on the satellites Explorer 45 and ATS 5 are used to infer the value of the dawn-dusk electric field present during a substorm. The observations were made during a substorm which occurred on December 17, 1971, while the two sathellites were in te evening local time sector, within the plasma sheet, and separated by ?1.5 RE in radial distance. A comparison of the variations at the two satellites shows that the substorm onset was observed at ATS 5?11 min before the onset at Explorer 45. We infer that a dawn-dusk electric field of ?11 kV/RE (?2 mV/m) was present during the substorm. We demonstrate that the observations made at the two satellites are compatible with the model of a uniform enhanced dawn-dusk electric field acting upon a particle source which is spatially uniform in the region from which particles are convected to synchronous orbit.