Progressive-ratio (PR) schedules of intravenous (i.v.) drug self-administration are useful for establishing the relationships between reinforcing effectiveness and pharmacological actions of abused drugs. The authors compared the reinforcing effects of the high-efficacy opioid alfentanil, the low-efficacy opioid nalbuphine, and cocaine using a PR schedule of i.v. drug injection in rhesus monkeys in which the response requirement increased during the experimental session and the initial response requirement (IRR) was varied. Analyses based on either consumer demand or labor supply models of behavioral economics revealed that the relative reinforcing effectiveness of cocaine and alfentanil was greater than that of nalbuphine. These results suggest that PR schedules with varying IRRs can provide meaningful estimates of the relative reinforcing effectiveness of abused drugs.