The motivation to self-administer heroin or cocaine was investigated using two different progressive ratio (PR) schedules of reinforcement. On the first schedule, the response requirements began at 1 and escalated systematically within the session. On the second schedule, the initial response requirements were adjusted according to the previous day's performance. Subsequent response ratios escalated exponentially. By comparing the breaking points derived from the two schedules, it was concluded that the motivation to self-inject cocaine increases with cumulative drug intake while the motivation to self-administer heroin decreases with each injection and is regulated by satiety. The ability of the second PR schedule of reinforcement to detect changes in the motivation to self-administer heroin was further investigated. Breaking points were shown to be sensitive to the opiate antagonist naltrexone. Heroin self-administration was shown to be remarkably resistant to extinction following saline substitution for heroin reinforcement although breaking points finally declined after several days. Finally, an inverted U-shaped dose-response relationship was established between breaking points reinforced with 12.5, 25, 50, and 100 |ig/inj unit heroin dose.