Mice did not respond for vehicle (dH2O + 30 μg/ml quinine (A). Fentanyl self-administration peaked on Days 17-19 for 1 μg/ml and Day 19 for 10 μg/ml but concentrations (B and C) 30 μg/ml and 300 μg/ml failed to induce opioid self-administration (D and E). Data shown is representative of mean lever presses for the fentanyl response bar (active lever) (triangles; FR1) and the control lever (circles). Analysis of the area under the curve (AUC) across all self-administration sessions indicated that 10 μg/ml of fentanyl (+ 30 μg/ml quinine) was the highest concentration of fentanyl that established self-administration behavior, showing statistically significant separation between the active and control levers (F) (*denotes a difference between active and respective control lever, Student’s paired t test, p< 0.05). N=8 mice per group.