Fig. 3. Influence of a continuous infusion of naloxone and placebo on 0.2 mg intravenous buprenorphine (  A ) and 0.4 mg intravenous buprenorphine (  B ). The buprenorphine dose was given over 60 min from t = 2 to t = 62 min (  light gray dots ). Naloxone or placebo was given for 2 h, from t = 32 to t = 152 min (  dark gray dots ).  Black circles = naloxone (n = 8 per treatment);  open circles = placebo (n = 8 per treatment). The  gray area represents the 95% confidence interval of predrug baseline ventilation. Mean ventilation data are relative to baseline.