During study months in 1981 and 1982, a total of almost 5,800 drivers were stopped on Friday and Saturday nights under laws in each country permitting the police to stop drivers at random to enforce the nation's drunk driving legislation. Included were 3,503 drivers from Melbourne, Victoria, Australia; 301 drivers from rural Victoria; and 1,987 drivers from Belfast, Northern Ireland. A total of 5.3 percent of the drivers in Melbourne, 2.1 percent of the drivers in rural Victoria, and 1.5 percent of the drivers in Belfast had used alcohol. A total of 8.5 percent of the Melbourne drivers, 2.6 percent of the rural Victoria drivers, and 5.5 percent of the Belfast drivers had used medication. In Melbourne, 12.7 percent of the drivers had used both alcohol and medication, compared to 3.7 percent of the rural Victoria drivers and 6.6 percent of the Belfast drivers. Differences between Melbourne and Belfast in the frequency of alcohol and medication use by drivers were generally consistent with differences in published data relating to the use of alcohol and medication in the two communities. Prescription drug use was reported up to six times more often than use of over-the-counter preparations. The community and existing drug control agencies therefore need to cooperate with road safety organizations in developing new countermeasures against driving under the influence of alcohol and other drugs. Maps, 39 data tables, and 116 references.