Figure 1.
Effects of the mu agonist fentanyl and the delta opioids SNC243A, MSF61 and naltrindole on rates of schedule-controlled responding. Abscissa: Dose of test drug in mg/kg (log scale). Ordinate: Percent control rates of responding. Each point shows mean ±S.E.M. for 5 monkeys (fentanyl, SNC243A) or 4 monkeys (MSF61, naltrindole).