Progressive visual reaction times were measured before and after exposure of 10 subjects (anesthetists or anesthetic technicians) to either halothane 100-150 ppm in 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 th subjects responses when breathing either air or a mixture of halothane 15 ppm an N2O 500 ppm in air (range: halothane 10.3-16.0 ppm, N2O 485-700 ppm).