Sixteen instrument-rated pilots, eight of whom were very experienced professional aviators, flew instrument landing system approaches in a Cessna 172 under simulated instrument flight conditions while sober and while under the influence of 40, 80, and 120 mg% of blood ethyl alcohol. Each pilot flew four approaches to minimums on each of two occasions at each alcohol level. The data collected during these approaches included continuous measurement of aircraft position with respect to localizer and glide path centerlines and airspeed. Note was made of procedural errors committed during the flights. It is concluded that even 40 mg% of blood alcohol exerts decremental effects on performance which are incompatible with flight safety.