Fig. 2. Antinociceptive effects (skin twitch response latency expressed in seconds with a 6-s maximum latency) measured before (baseline) and at 1, 6, and 24 h after beginning a 6-h intrathecal infusion (40 μl/min) of three concentrations (mg/ml) of d/l-methadone (n = 3), l-methadone (n = 3), or d-methadone (n = 2).