Comparison of fixed-ratio and progressive-ratio schedules of maintenance of stimulant drug-reinforced responding

The effectiveness of doses of i.v. cocaine and nomifensine in maintaining lever-press responding in rhesus monkeys was evaluated under two schedules, fixed- and progressive-ratio (FR, PR). The doses that maintained maximum rates of responding under the fixed-ratio schedule were 0.32 mg/kg per injection cocaine and 0.10 mg/kg per injection nomifensine. The fixed-ratio rates maintained by this dose of nomifensine were slightly lower than those maintained by cocaine. Under the progressive-ratio schedule, the maximum response rates developed with 0.32 mg/kg per injection cocaine and 0.32 mg/kg per injection nomifensine. Maximum performances under the progressive ratio were slightly higher with cocaine than with nomifensine. Taken in conjunction with existing data for other drugs and conditions, these data indicate that progressive-ratio schedules may yield information on the relative reinforcing effects of drugs that differs only slightly from that obtained with fixed-ratio schedules.