Fig. 3. Pharmacokinetic–pharmacodynamic relations of unbound oxycodone (Δ) and morphine (▴) in brain  (A ) and unbound oxycodone (□) and morphine (▪) in blood  (B ). The average observed latencies with SEMs are plotted against the average unbound concentrations in brain and blood, predicted by the pharmacokinetic model, with SEMs. The predicted concentrations at the time of tail-flick latency measurements were used in the plots, as the concentration measurements were not made at exactly the same times as the measurements of tail-flick latency.