(A) Mean (±SEM) number of active lever presses for ShA and LgA females and males following vehicle, 10 μg/kg fentanyl, and 30 μg/kg fentanyl. All rats reinstated to 10 μg/kg and 30 μg/kg fentanyl in comparison to vehicle *(p < .05). The difference between vehicle and 30 ug/kg fentanyl responding was greater for the LgA group than the ShA group ^(p < .05); note the extreme outlier ShA female following 30 μg/kg fentanyl. Females had higher active lever pressing than males overall +(p < .01). (B) Mean (±SEM) number of active lever presses for ShA and LgA females and males following vehicle, 1 mg/kg yohimbine, and 2 mg/kg yohimbine. The 1 mg/kg yohimbine dose produced reinstatement in comparison to vehicle, collapsed across access group and sex *(p < .001). Note that in both figure panels, presses on the active lever during the time-out period were not included.