Calculated ED50 values (mg/kg) from tail-flick tests following treatment with WIN55,212-2 and CP55,940. ED50 values were calculated from dose-response curves generated by non-linear regression analysis. Values shown are mean and 95% confidence interval, and 10–12 mice were tested for each group. Potency ratios were calculated as the ratio between the ED50 after chronic drug dosing and the ED50 prior to chronic dosing. Confidence intervals that could not be accurately calculated from curve fitting are listed as not able to determine (ND). Data were analyzed using F tests.