Respiratory and metabolic functions were studied at rest and during exercise in 13 Olympic oarsmen at peak of training at sea level and after one month's residence at 2350 m. At sea level each subject completed two severe five minute treadmill runs at 5 mph on a 20% incline inspiring 21% O2for one run and 16% O2for the other. Two more runs at the same speed and incline were carried out at altitude with F102's of 21% and 28%. Pulmonary function tests were carried out at sea level and altitude and steady state diffusion capacity was determined during rest and exercise while breathing 21% O2. Heart rates were monitored by direct electrocardiography. At altitude significant increases were found in MVV (10%), FEV1(6%), MMEF (15%) and PF (9%) hut not in VC. A rise of the DLCOduring exercise from 64.8 to 75.4 ml/min/mm Hg was largely the result of increased ventilation. The response to acute hypoxia (16% O2) and to chronic hypoxia were as follows: VE from 113 to 135/122 1/min; VE/O2from 2.55 to 3.14/3.08 1/100ml; and Petco2 from 40 to 36/34 min Hg. The effect of chronic hypoxia could be abolished by acute normoxia (28% O2) within five minutes; Ve: returned to 99 l/min, Ve/VO2to 2.46 1/100 ml, and PETCO2to 40 mm Hg. VO2for the standard exercise (5 mph − 20% grade) for normoxic and acute hypoxic conditions were similar; 4421 and 4301 ml/min, but this variable decreased significantly upon chronic altitude exposure; 3966 ml/min. This decrement in Vo2was attributed in part to a lower work of breathing.