1. Twenty normal subjects, twenty patients with emphysema and twenty with anaemia breathed 0·03% carbon monoxide for 45 min. For all sixty the mean increase in carboxyhaemoglobin (COHb) was 4·5%. Patients with emphysema showed a significantly smaller increase than did the other two groups.

2. The mean decrease in oxygen content was 209 vol./100 ml and was significantly less in patients with emphysema than in the other two groups.

3. All sixty subjects showed a significant diminution in reaction speed to a visual stimulus, but no significant change in tests for depth perception and visual discrimination for brightness.

4. No differences among the three groups were observed in respect to these psychological tests.

5. It is concluded that COHb concentrations of 5% may adversely affect reaction time to visual stimuli and that patients with emphysema are less susceptible to accumulation of COHb and decrease in venous oxygen content than normal subjects or patients with anaemia.

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