We examined the efficacy of oral methadone as a reinforcer by offering methadone maintenance patients the chance to self-administer extra doses of methadone occasionally in addition to their regular dose. Seven maintenance patients received twice-weekly choices between methadone doses or money. Doses were 0, 1, 5, 10, 25, or 50 mg methadone; the alternative money option was $1 or $5. Extra methadone doses were reliably self-administered by maintenance patients, and percent of dose choices rose as the size of the dose offered increased. Thus extra methadone doses functioned as reinforcers in this situation. Further, across the entire dose range, more dose choices were selected when $1 was offered than when $5 was offered as an alternative. Thus methadone self-administration was influenced by the alternative reinforcers available for drug refusal. Neither reports of subjective withdrawal symptoms nor reduction of symptoms after extra methadone predicted methadone self-administration, but dose selections were more likely when urinalysis results indicated recent illicit opiate use. The reinforcing effects of oral methadone in methadone-tolerant patients may be an important factor in the popularity of this treatment among drug abusers and in the long-term treatment retention generally observed during methadone maintenance.