G protein–coupled receptors, in particular, Ca²⁺-mobilizing G_q-coupled receptors have been reported to be targets for anesthetics. Opioids are commonly used analgesics in clinical practice, but the effects of anesthetics on the opioid μ-receptors (μOR) have not been systematically examined. We report here an electrophysiological assay to analyze the effects of anesthetics and ethanol on the functions of μOR in Xenopus oocytes expressing a μOR fused to chimeric Gα protein G_qi5 (μOR-G_qi5). Using this system, the effects of halothane, ketamine, propofol, and ethanol on the μOR functions were analyzed. In oocytes expressing μOR-G_qi5, the μOR agonist DAMGO ([D-Ala²,N-MePhe⁴,Gly-ol]-enkephalin) elicited Ca²⁺-activated Cl⁻ currents in a concentration-dependent manner (EC₅₀ = 0.24 μM). Ketamine, propofol, halothane, and ethanol themselves did not elicit any currents in oocytes expressing μOR-G_qi5, whereas ketamine and ethanol inhibited the DAMGO-induced Cl⁻ currents at clinically equivalent concentrations. Propofol and halothane inhibited the DAMGO-induced currents only at higher concentrations. These findings suggest that ketamine and ethanol may inhibit μOR functions in clinical practice. We propose that the electrophysiological assay in Xenopus oocytes expressing μOR-G_qi5 would be useful for analyzing the effects of anesthetics and analgesics on opioid receptor function.