This survey analyzes the concentration-effect relationship of BZDs in field conditions in terms of performance on the clinical test for drunkenness (CTD), which is a set of motor, vestibular, mental, and behavioral subtests developed in Finland to examine drunk drivers. The CTD is administered only to drivers who are suspected of driving under the influence of drugs other than alcohol. To reveal drug effects on CTD performance, blood and urine samples of 1,069 Finnish drivers drawn between January 1985 and November 10, 1986 were analyzed for BZDs and alcohol. Final analysis involved 327 cases. Impairments on the CTD correlated significantly both to blood alcohol concentration and to blood BZD concentration. The results confirm the sensitivity of the CTD in revealing alcohol-induced impairments of psychomotor performance, and also show a clear concentration-response relationship between CTD performance and blood BZD concentrations. Tables, references