Fermilab E866 has performed a precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on hydrogen and deuterium targets, leading to the first determinations of d/ and d - in the proton as functions of x. The results show that d > over a broad range of x and provide valuable information regarding the origins of the d/ asymmetry and the antiquark sea in the nucleon.