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Effects of the opioid receptor agonists alfentanil (closed circles) and nalbuphine (open squares) on number of injections/session and break point under a progressive-ratio schedule. A 30-min timeout followed each injection. The initial response requirement was 100, and this requirement doubled every four injections. Each point represents data averaged for a group of four rhesus monkeys. Point over “S” shows performance when saline was available for self-administration.
