  The Fermilab E866/NuSea Collaboration has measured the Drell-Yan dimuon cross
sections in 800 GeV/$c$ $pp$ and $pd$ collisions. This represents the first
measurement of the Drell-Yan cross section in $pp$ collisions over a broad
kinematic region and the most extensive study to date of the Drell-Yan cross
section in $pd$ collisions. The results indicate that recent global parton
distribution fits provide a good description of the light antiquark sea in the
nucleon over the Bjorken-$x$ range $0.03 \lesssim x < 0.15$, but overestimate
the valence quark distributions as $x \to 1$.
