Fig. 1. 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 of morphine, hydromorphone, fentanyl, or DAMGO. Each curve presents the mean and SEM of 3 dogs. All doses are given as mg/kg.