Zopiclone is a frequently prescribed hypnotic drug. Related to the therapeutic effects is impairment of cognitive functioning and psychomotor skills. The thesis is based on a double blind, placebo-controlled, crossover, randomized trial in 16 male subjects. The subjects received 5 mg zopiclone, 10 mg zopiclone, 50 g ethanol or placebo on different study days. On each study day pairs of Oral Fluid (OF)/blood were retrieved and several simplified clinical and computerized tests were performed. We found dose/concentration-related impairment of both zopiclone and ethanol for the clinical and the computerized tests. The computerized tests revealed more impairment than the clinical tests. Tests that measure reaction time were more likely to be influenced by zopiclone, tests that measure impulsive responses were more likely to be affected by ethanol. Acute tolerance to tests that measure automative behavior and or reaction time was found for zopiclone. The zopiclone concentration in OF and the OF/blood concentration ratio was dependent on several variables, such as OF sampler device applied, amount of OF delivered and intake of food. We detected zopiclone in OF up to 14 days after intake.