Opioid peptides morphiceptin (Tyr-Pro-Phe-Pro-NH2), [D-Ala2, D-Leu5]-enkephalin (Tyr-D-Ala-Gly-Phe-D-Leu, DADLE) and [D-Thr2]-Leu-enkephalin-Thr (Tyr-D-Thr-Gly-Phe-Leu-Thr, DTLET) containing tyrosine has been studied in the reaction with mushroom tyrosinase immobilized on Clark-type oxygen electrode. A 4–6-times higher response is observed for tyrosine compared to peptides. The detection limit of the tyrosinase-modified electrode for DADLE, morphiceptin and DTLET was 6 μM, 9 μM and 16 μM, respectively.