This study examined the effects of a presumed neurotoxic dose regimen of (+)-fenfluramine on the discrimination of MDMA and (+)-amphetamine in male Sprague-Dawley rats trained to discriminate 1.5 mg/kg MDMA from saline in a two-choice operant task. Substitution tests were conducted with saline, several doses of MDMA (0.19-1.5 mg/kg), and (+)-amphetamine (0.125-1.0 mg/kg) prior to and again following the administration of (+)-fenfluramine (4.0 mg/kg twice a day for 4 days; n = 11) or a similar pattern of saline injections (n = 10). During pretreatment substitution tests, lower doses of MDMA elicited drug-appropriate responding in a dose-dependent manner, although none of these doses substituted for the training dose. Likewise, no dose of (+)-amphetamine substituted for the training drug during pretreatment substitution tests. The discrimination of MDMA was disrupted in some animals following (+)-fenfluramine treatment, but with subsequent training, discrimination criteria were met. In posttreatment substitution tests, the lowest dose of MDMA produced significantly higher drug-appropriate responding in (+)-fenfluramine treated animals but not in saline-treated animals. The amount of drug-appropriate responding during posttreatment substitution tests with (+)-amphetamine varied little from pretreatment substitution tests in saline-treated animals, but was greater at all doses in (+)-fenfluramine-treated animals; the highest dose of (+)-amphetamine substituted for MDMA subsequent to (+)-fenfluramine treatment. These results support previous findings that the long-lasting serotonergic effects of fenfluramine may have functional consequences that can be detected using a drug discrimination procedure. Specifically, serotonin depletion may unmask or strengthen the stimulant-like effects of MDMA.