Various tests of psychomotor performance were carried out in a group of 16 healthy male volunteers given a standardized preparation of Korean ginseng (G 115; 100 mg twice a day for 12 weeks) and in a similar group given identical placebo capsules under double-blind conditions. A favourable effect of G 115 relative to baseline performance was observed in attention (cancellation test), processing (mental arithmetic, logical deduction), integrated sensory-motor function (choice reaction time) and auditory reaction time. However, end performance of the G 115 group was superior statistically to the placebo group only in mental arithmetic. No difference between G 115 and placebo was found in tests of pure motor function (tapping test), recognition (digit symbol substitution) and visual reaction time. No adverse effects were reported. It is concluded that G 115 may be superior to placebo in improving certain psychomotor functions in healthy subjects.