Laboratory studies of field sobriety tests, which were conducted during the 1970s, identified horizontal gaze nystagmus (HGN) as a sensitive and reliable marker of alcohol at BACs of 0.10 per cent and above. In a more recent study, McKnight, Langston, Lange, and McKnight found that HGN remains a valid field sobriety test at 0.08 per cent BAC. The observational threshold of HGN, however, has not been established. A study was undertaken to further examine HGN at 0.08 per cent BAC and to determine the lowest BAC at which its signs appear reliably. Measurements of the angle of gaze (AOG) at the onset of nystagmus were obtained with 48 subjects, who had been given alcohol to produce peak BACs in the range 0.02 per cent - 0.12 per cent. Observations were made with the EM/1, an instrument which displays a magnified video image of subjects eyes. The obtained data provide further evidence of the reliability of HGN as an index of the presence of alcohol at 0.08 per cent. Further study will be required to establish a BAC threshold for HGN.