Measurements of the polar cap potential drop and size have been obtained during magnetospheric substorms. Using double-probe electric field measurements on the DE 2 satellite, 148 measurements have been obtained at random times preceding, during, and after 64 substorms. The polar cap potentials are graphed as a function of the difference between the time of the polar cap measurement and the time of the expansion onset of the corresponding substorm. The ratios of the auroral electrojet (AE) indices and the potential are also determined. The results show that on the average the polar cap potential starts to increase at 1.5 hours before onset, reaches a level of 70 kV in the 30-min period before onset, and starts to decline at 1.5 hours after onset. However, on a case-by-case basis there are substantial variations from the average, as polar cap potentials over 120 kV were measured as early as 1 hour before substorm onset and values as low as 40 kV were observed during the expansion phase. The size of the polar cap ranged from 23¿ to 38¿ invariant latitude at the time of onset, and had an average value of 31¿. The AE/&PHgr;PC ratio is nearly constant before and after substorms, but decreases slightly during the substorm growth phase and increases greatly during the expansion phase. This increase is most likely due to a higher conductivity and westward electric field within the electrojet during expansion, which causes AE to increase without a corresponding change in the polar cap potential. ¿ American Geophysical Union 1992