Four opioid peptides were isolated from the enzymatic digest of wheat gluten. Their structures were Gly-Tyr-Tyr-Pro-Thr, Gly-Tyr-Tyr-Pro,Tyr-Gly-Gly-Trp-Leu and Tyr-Gly-Gly-Trp, which were named gluten exorphins A5, A4, B5 and B4, respectively. The gluten exorphin A5 sequence was found at 15 sites in the primary structure of the high molecular weight glutenin and was highly specific for delta-receptors. The structure-activity relationships of gluten exorphins A were unique in that the presence of Gly at their N-termini increased their activities. Gluten exorphin B5, which corresponds to [Trp4,Leu5]enkephalin, showed the most potent activity among these peptides. Its IC50 values were 0.05 microM and 0.017 microM, respectively, on the GPI and the MVD assays.