The effect of a standard daily regimen of chlordiazepoxide, prazepam (a new benzodiazepine tranquilizer), and placebo were examined in a three way double-blind comparison for a sample of normal volunteers. The criteria include a limited spectrum of psychomotor functions. A learning effect was conspicuous for all drugs on all criteria during the day-long sequence of ten trials. For the most part the differences between the drug groups were insignificant, but there were distinctive modifications in the performance of the chlordiazepoxide group. The clearest effect of the tranquilizer medications was found in the time estimation tests. Paradoxically, the drugs correct a naturally occurring perceptual error.