Laboratory analysis has an important role in detecting drugged drivers and in obtaining successful prosecutions. From March 1983 to October 1985, 163 positive cases were handled by the Forensic Science Laboratory in Perth, Western Australia. A total of 30 drugs were detected, the most frequently encountered being cannabis, the benzodiazepines (predominantly oxazepan), and morphine/codeine, which in most cases is present probably as a metabolite of heroin. Cannabis has been found to impair driving ability seriously. Police will not request blood and urine samples unless there is strong visual evidence, and they will not proceed with a prosecution without sufficient supporting visual evidence. Thus, most people whose body fluid samples are tested plead guilty. 5 tables.