In view of the widespread application of methylphenidate for attention-deficit/ hyperactivity disorder (ADHD) therapy its interaction with alcohol was investigated in an in-vitro assay and in a study involving 9 male volunteers. The study conditions were: methylphenidate (20 mg) only, methylphenidate followed by ethanol (0.8 g/kg body weight) and ethanol followed by methylphenidate. Methylphenidate (CAS 113-45-1), ritalinic acid (CAS 19395-41-6) and ethylphenidate (CAS 57413-43-1) were assayed in blood samples collected up to 7 h after ingestion using liquid chromatography-mass spectrometry (LC/MS). It was found that methylphenidate is hydrolyzed to ritalinic acid by the same esterase that degrades cocaine. In the presence of ethanol this is inhibited and the active metabolite ethylphenidate is formed. The pharmacokinetic evaluation showed that methylphenidate concentrations were not markedly affected by ethanol, but ritalinic acid concentrations were lower, especially if ethanol was ingested first. Ethylphenidate concentrations were low with only about 10% of methylphenidate concentrations suggesting that concurrent ethanol use does not impair methylphenidate's therapeutic efficacy. Unexpectedly one subject exhibited a methylphenidate hydrolysis defect yielding very high methylphenidate and low ritalinic acid concentrations in all study conditions.