SUMMARY1. Six international middle-distance runners were investigated during 4 weeks in England and during a similar period in Mexico City (2270 m (7450 ft.)) 2. In 3-mile (4828 m) time trials at 2270 m the increase in time taken by four subjects compared with sea level was 8-5 % on the 4th day and 5-7 % on the 29th day. There was thus a gain of 2-8 % or 20 sec in time associated with acclimatization.3. In 1-mile (1609 m) time trials the times were increased by 3*6 % in the first week at altitude and by 1F5 % in the 4th week. The improvement amounted to 2-1 %, or 4-9 sec.4. In 5 min maximum exercise on the ergometer maximum 02 intake for six subjects at altitude was reduced by 14x6 % on the 2nd day and 9*5 % on the 27th. Only one subject showed no change in maximum oxygen intake ( Vo2, max) with time spent at altitude. 5. Although 802 m2ax was persistently reduced at altitude work rates finally exceeded sea-level values, owing to increased over-all efficiency.6. Forty-minute recovery 02 intakes after 5 min maximum exercise averaged 17-35 1. at sea level and 17-53 1. at altitude. Mean values from 40th to 50th min were within + 7 % of pre-exercise values.7. Serial tests at increasing loads yielded a straight-line relation between 02 intake and work rate over a wide range of work rates at sea level and at altitude. Heart rate and ventilation for given work intensity were maximal in the first 2-10 days at altitude and thereafter declined.8. Capillary HbO2 saturation fell from 93 % at rest to 87 % in maximum exercise. The corresponding alveolar gas tensions were PA,°8 9 mm Hg, PA, Co 24 mm Hg. About half the total unsaturation in maximum exercise was explained by the Bohr effect.9. In six of eight pairs of determinations JX02, max measured on the ergometer was within + 0.15 1./min of J%02, max measured on the running track. Nevertheless, it was not possible to predict running performance from ergometer measurements.