Methylphenidate has potential for abuse because it produces behavioural effects similar to those observed with other abused stimulants, such as d-amphetamine and cocaine. The aim of this study was to further characterize the abuse potential of oral methylphenidate relative to oral d-amphetamine. Ten drug-abusing volunteers were recruited to participate in this study, which consisted of seven dose conditions: methylphenidate (16, 32 and 48 mg), d-amphetamine (8, 16 and 24 mg) and placebo. The reinforcing effects of these drugs were assessed during a self-administration session (preceded by a sampling session for each condition) with a modified progressive-ratio procedure. Subject-rated, performance and physiological effects were assessed concurrently during both the sampling and self-administration sessions. The intermediate dose of methylphenidate and d-amphetamine increased responding significantly above placebo levels. Both methylphenidate and d-amphetamine produced dose-dependent increases in stimulant-like subject ratings (e.g. Active, Alert, or Energetic and High), but the effects of these drugs were not isomorphic. These findings are consistent with epidemiological data and previous findings from laboratory studies that suggest methylphenidate has at least some abuse potential.