Figure 1. Minimum alveolar concentration (MAC) for halothane as a function of the dose of the competitive glycine receptor antagonist 5-nitro-6,7-dichloro-2,3-quinoxalinedione (ACEA-1021). Each point depicts values for an individual rat. ACEA¹= ACEA-1021 bolus 20 mg/kg intravenously, infusion 14 mg *symbol* kg sup -1 *symbol* h sup -1; ACEA²= ACEA-1021 bolus 40 mg/kg intravenously, infusion 14 mg *symbol* kg sup -1 *symbol* h sup -1. Mean plus/minus SD MAC for halothane: control group, 0.95 plus/minus 0.15 vol%; ACEA, 0.50 plus/minus 0.14 vol%; ACEA², 0.14 plus/minus 0.16 vol%. Pearson correlation coefficient, r = -0.92; t = 9.31; P = 3 x 10⁸.