  Deep inelastic $e^-p$ scattering has been studied in both the charged-current
(CC) and neutral-current (NC) reactions at momentum transfers squared, $Q^2$,
between 400 GeV$^2$ and the kinematic limit of 87500 GeV$^2$ using the ZEUS
detector at the HERA $ep$ collider. The CC and NC total cross sections, the NC
to CC cross section ratio, and the differential cross sections, $ d\sigma/dQ^2
$, are presented. For $Q^2 \simeq M_W^2$, where $M_W$ is the mass of the $W$
boson, the CC and NC cross sections have comparable magnitudes, demonstrating
the equal strengths of the weak and electromagnetic interactions at high $Q^2$.
The $Q^2$ dependence of the CC cross section determines the mass term in the CC
propagator to be $M_{W} = 76 \pm 16 \pm 13$~GeV.
