The 1967 Road Safety Act specified for the first time legal limits to the amount of alcohol a driver could possess in the blood or urine and introduced the use of roadside breathing devices to help police officers. The law worked effectively for only a few years, and by 1974 a working party was established to review the legislation. Among its recommendations was a suggestion to look at new instrumentation and evaluate the use of breath alcohol instruments. After field trials in 1977-78 showed that evidential breath testing was a practical possibility, Parliament passed the Transport Act in 1981 which allowed breath tests to be used in court proceedings in place of blood or urine analysis. Field trials of three devices were conducted in 1981 using over 1,500 subjects. These researchers decided that the instrument's calibration should be checked immediately before and immediately after the subject gave the required two breath samples and then chose liquid simulators for this purpose. The two instruments selected for use under the new law were the Camic Breath Analyser and the Lion Intoximeter because of their excellent specificity, precision, and accuracy. The police forces have ordered about 700 instruments for delivery in early 1983 and plan to train about 9,000 operators by that time. This scheme will allow motorists to be processed at the police station expeditiously and improve the overall speed of dealing with the drinking driver. Graphs and four references are supplied.