Progressive visual reaction times were measured before and after exposure of 10 subjects (anaesthetics 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 485-700 p.p.m.).