Nitrous oxide and halothane, in concentrations as low as 50 p.p.m. and 1.0 p.p.m. respectively, caused measurable decrements in performance in psychological tests in healthy male volunteers. Nitrous oxide alone caused similar effects. The functions apparently most sensitive to these low concentrations of anaesthetic agents were visual perception, immediate memory, and a combination of perception, cognition and motor responses required in a task of divided attention to simultaneous visual and auditory stimuli. These effects were absent in subjects exposed to nitrous oxide 25 p.p.m. and halothane 0.5 p.p.m.