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The present study examined whether in humans the generalized matching law described the relation between relative responding and relative drug intake by humans under concurrent variable interval variable interval (conc VI VI) schedules of drug reinforcement. Methadone-maintained patients, stabilized on 80 mg per day of methadone, were recruited and trained to button press for repeated deliveries of small volumes (10 ml) of 0.08 mg/ml methadone solution. In the training phase, deliveries of methadone or vehicle solution were arranged under conc VI VI schedules of reinforcement. The mean interval for the methadone and for the vehicle options was 60, 90, 120, 180, and 240 s. During another phase, responding on either of 2 buttons produced methadone solution. For the comparison option, the mean interval was 60, 90, 120, 180 or 240 s. For the concurrently available standard option, the mean interval was a constant 120 s. When methadone and vehicle were available, methadone was preferred to vehicle. When methadone was available at either option, the generalized matching law described the relation between relative response allocation and methadone intake. The results extend the generality of the matching law to human drug self-administration. The study also demonstrated the importance of reinforcement context as a determinant of human behavioral allocation.