The acute and sub-chronic effects of moclobemide and mianserin on driving and psychometric performance were compared to those of placebo in a double-blind, cross-over study involving 17 healthy volunteers. Mianserin, moclobemide and placebo were administered for 8 days. Subjects' performance was measured on days 1 and 8 of each treatment series; subjective sleep parameters. mood. and possible side-effects were recorded each treatment day on questionnaires or visual analog scales. Mianserin affected most of the performance measures, while moclobemide affected none; mianserin also impaired driving and tracking performance and decreased CFF. Whilst receiving mianserin, subjects reported depressed levels of alertness, calmness, and contentment; the quality of their sleep was unaffected, but its duration increased, together with feelings of drowsiness and fatigue during the day. No statistical interactions between the factors Drugs and (Treatment) Days were found, indicating that little pharmacological tolerance developed over time during mianserin treatment. Mianserin's sedative properties are held responsible for all performance and subjective effects of the drug. It is concluded that moclobemide 200 mg b.i.d. has no important sedative properties.