Groups of mice (n ≥ 20) were injected subcutaneously with a fixed ratio of the stated drug and l-methadone and tested 30 min later for analgesia, with the exception of fentanyl, which was given 20 min after l-methadone injection (10 min before testing) in order for the peak effects to coincide with those ofl-methadone. The following methadone/drug ratios, on a weight basis, were used: morphine, 0.5:1; M6G, 0.5:1; codeine, 0.5:1; 6-acetylcodeine, 9.5:1; fentanyl, 95:1. Values for the ED₅₀ ± S.E.M. were determined using regression probit analysis, as described under Materials and Methods. The d-isomer of methadone showed no analgesia at a dose equivalent to the ED₈₀of l-methadone. In the combinations, both the observed and the predicted additive total doses of both drugs together are presented. ED₅₀ values for the combination and the predicted additive values represent the total dose of the two drugs together. The amount of each component represents its fraction of the total dose, based upon the ratios of the drugs used to generate the dose-response curve. The enhanced potency, indicative of synergism, is indicated by a significant difference between the additive and combination ED₅₀ values using ANOVA. All shown combinations, with the exception of fentanyl, indicated a significant potency enhancement. Isobolograms, with insets and confirming ANOVA tests, are shown in Fig.3.