Progressive visual reaction times were measured before and after exposure of 10 subjects (anaesthetists or anaesthetic technicians) to either halothane 100–150 p.p.m. in air or air alone. There were no significant differences between the control and test situations. In a subsequent study of 15 psychology students using an audiovisual reaction time test, a tachistoscope and the Wechsler memory scale, there was no significant difference in the subjects' responses when breathing either air or a mixture of halothane 15 p.p.m. and nitrous oxide 500 p.p.m. in air (range: halothane 10.3–16.0 p.p.m., nitrous oxide 485–700p.p.m.).