Fig. 2. The effect of placebo (A ), 25 μg naloxone (B ), 100 μg naloxone (C ), and 400 μg naloxone (D ), all given at t = 55 min on morphine-6-glucuronide (M6G)-induced respiratory depression. The data are mean ± SEM. Ventilation is relative to baseline values. To guide the eye, the baseline ventilation value and ventilation at t = 50 min are connected via dotted lines . The arrows  represent the infusion of M6G, placebo (P), or naloxone (N).