The effect of hypoxaemia (mean SpO2 78%) on cognitive function was measured in two groups of twelve normal subjects. A series of psychometric tests was administered to each subject in the same sequence and consisted of the Reitan trail-making test, a digit symbol substitution test, a visuospatial orientation test and the simple unprepared reaction-time test. Psychomotor performance was assessed in a double-blind manner while the subjects were breathing first air and then either air or a hypoxic mixture. While there was improvement in time for the trail-making test during a repeat study breathing air, there was significant deterioration of time to completion of the test in conditions of hypoxia. A significant learning effect in the orientation test was seen in the control group but this did not occur in hypoxic subjects. Hypoxaemia was shown to cause a significant impairment of simple unprepared reaction time compared with controls. All the changes in cognitive function were small and there were no subjective differences in the air or hypoxic groups. The usefulness of the Reitan trail-making and the simple unprepared reaction-time test in the assessment of psychomotor performance deficit under conditions of hypoxaemia has been demonstrated by this study in normal subjects. It was concluded that a mean oxygen saturation of 78% caused only minor changes in cognitive function in normal subjects.