In Western societies, a considerable percentage of young people use 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy'). The use of alcohol (ethanol) in combination with ecstasy is common. The aim of the present study was to assess the acute psychomotor and subjective effects of (co-) administration of MDMA and ethanol over time and in relation to the pharmacokinetics. We performed a four-way, double blind, randomized, crossover, placebo-controlled study in 16 healthy volunteers (nine men, seven women) between the ages of 18 and 29. MDMA (100 mg) was given orally while blood alcohol concentration was maintained at pseudo-steady state levels of approximately 0.6 per thousand for 3 h by a 10% intravenous ethanol clamp. MDMA significantly increased psychomotor speed but did not affect psychomotor accuracy and induced subjective arousal. Ethanol impaired both psychomotor speed and accuracy and induced sedation. Coadministration of ethanol and MDMA improved psychomotor speed but impaired psychomotor accuracy compared with placebo and reversed ethanol-induced sedation. Pharmacokinetics and pharmacodynamics showed maximal effects at 90-150 min after MDMA administration after which drug effects declined in spite of persisting MDMA plasma concentration, with the exception of ethanol-induced sedation, which manifested itself fully only after the infusion was stopped. In conclusion, results show that subjects were more aroused when intoxicated with both substances combined compared with placebo, but psychomotor accuracy was significantly impaired. These findings may have implications for general neuropsychological functioning as this may provide a sense of adequate performance that does not agree with a significant reduction in psychomotor accuracy.