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Self administration of heroin following termination of morphine administration. Heroin was available for self administration every day and data were averaged across days for 2 weeks to generate the dose–effect curves shown. The number of infusions received in each of the five cycles with different unit doses of heroin available during each cycle (ordinate) is plotted as a function of unit dose of drug (abscissae) expressed in micrograms per kilogram per injection
