Etorphine, the most potent narcotic analgesic known, was labeled with tritium by catalytic exchange. This drug exhibits stereospecific, saturable binding to rat-brain homogenate. At saturation, the stereospecific binding is 0.1-0.15 pmol/mg of protein. Specific binding is inhibited high salt concentrations, sulfhydryl reagents, and proteolytic enzymes, but is unaffected by phospholipases A and C, sodium azide, sodium fluoride, and prostaglandins E(1) and E(2). Competition for binding of [(3)H]etorphine correlates with agonist and antagonist potencies. The stable, stereospecific binding of an active narcotic analgesic supports the existence of opiate receptors.