Tritiated porcine α-neo-endorphin has been prepared from its corresponding iodinated analog. The iodinated analog (diiodotyrosine at position 1) was synthesized, along with its non-iodinated counterpart, by the solid-phase method. Catalytic exchange of this iodinated analog in the presence of tritium yielded tritiated porcine α-neo-endorphin having a specific activity of 45.5 Ci/mmole. Both the native, iodinated and tritiated α-neo-endorphin analogs were shown to be homogenous by chromatography on carboxymethylcellulose, paper chromatography, paper electrophoresis, high performance liquid chromatography and amino acid analysis. For the first time binding of α-neo-endorphin to rat membrane preparations is described using [ 3H 2Tyr 1]α-neo-endorphin as the ligand. The binding is time-dependent and saturable with respect to α-neo-endorphin. Scatchard analysis was bi-phasic with K Ds of 0.20 and 3.75 nM. Displacement binding studies indicate that the receptor for α-neo-endorphin has “kappa” and possibly “epsilon” binding characteristics.