1. 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 1.5% in the 4th week. The improvement amounted to 2.1%, or 4.9 sec.4. In 5 min maximum exercise on the ergometer maximum O(2) intake for six subjects at altitude was reduced by 14.6% on the 2nd day and 9.5% on the 27th. Only one subject showed no change in maximum oxygen intake (V(O2, max)) with time spent at altitude.5. Although V(O2, max) was persistently reduced at altitude work rates finally exceeded sea-level values, owing to increased over-all efficiency.6. Forty-minute recovery O(2) intakes after 5 min maximum exercise averaged 17.35 l. at sea level and 17.53 l. 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 O(2) 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 HbO(2) saturation fell from 93% at rest to 87% in maximum exercise. The corresponding alveolar gas tensions were P(A, O2) 89 mm Hg, P(A, CO2) 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 V(O2, max) measured on the ergometer was within +/- 0.15 l./min of V(O2, max) measured on the running track. Nevertheless, it was not possible to predict running performance from ergometer measurements.