Fig. 1.

Discriminative stimulus effects of naltrexone in three morphine-treated monkeys discriminating 0.0178 mg/kg naltrexone while responding under a fixed-ratio 5 schedule of SST. Saline was administered on the first cycle. Data shown are from five determinations of naltrexone dose–effect curves for each monkey (mean±SEM). The SEM does not appear if it was less than the radius of the data point. Ordinate: percentage of responses occurring on the naltrexone lever (%DR drug responding, top) and mean response rate expressed as percentage of control rate (saline training days, bottom). Abscissa: naltrexone dose in milligrams per kilogram of body weight (V vehicle)