Behavior Maintained Under Fixed-Interval and Second-Order Schedules of Cocaine or Pentobarbital in Rhesus Monkeys1

mm) or a second-order Fl 5 mm schedule of fixed-ratio 1 0 brief stimuli components [Fl 5 mm (FR 1 O:S)] by cocaine (0.0125-1.0 mg/kg/infusion) or pentobarbital (0.05-3.0 mg/kg/infu-sion) delivery during daily 3-hr sessions. Rates and patterns of responding were similar with both drugs, although cocaine was 1 0 to 64 times more potent than pentobarbital. As dose was increased, rates of responding increased to a maximum and then decreased under both schedules. For both drugs, rates of responding were variable from session to session as well as over successive intervals, but performance was generally typ-ical of interval schedules. The variability in responding over successive intervals was least for cocaine under the Fl sched-ule and greatest for pentobarbital under the Fl (FR) schedule. Drug intake was a direct function of dose and the number of infusions per session decreased as dose was increased. A