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Oxygen delivery (DO2) calculated from cardiac output, haematocrit (Hct) and arterial oxygen saturation (SaO2), has been obtained on six subjects at sea level (London) and after slow ascent to 5000 meters (Chamlang base camp) at rest and during mild exercise (25 watts and 50 watts). Haematocrit was increased in all six subjects at 5000 m and expressed as haemoglobin (Hb) rose from a mean (+/- standard error; SEM) of 13.8 +/- 0.1 g (100 ml)(-1) to 15.8 +/-0.3 g (100 ml)(-1) (t = 6.3, p = 0.0014). SaO2 was almost constant with exercise at sea level (rest 98.5%, 25 w 98.3% and 50 w 98.3%) but declined more steeply with exercise at 5000 m (rest 88.8 +/-0.6%, 25 w 85.4 +/-0.4% and 50 w 84.4 +/- 0.5%). Arterial oxygen content (CaO2) was very similar for 25 watts exercise at altitude (5000 m, 18.1 ml per decilitre--dl) as at sea level (London, CaO2 18.2 ml dl(-1)). At rest CaO2 was higher at altitude (18.8 +/-0.2 ml dl(-1)) than at sea level (18.3 +/- 0.4 ml dl(-1)) and at 50 w CaO2 was lower at altitude (17.9 +/- 0.4 ml dl(-1)) than at sea level (18.2 +/- 0.2 ml dl(-1)). Hence, similar cardiac output values at rest (sea level, 5.0 +/- 0.4 litres min(-1) l min(-1); altitude, 5.6 +/- 0.31 min(-1)-) and at 25 w exercise (sea level, 8.2 +/-0.7 1 min(-1); altitude, 8.3 +/-0 .9 1 min'(-1) resulted in similar values for DO2 at rest (sea level, 0.9 +/-0.1 l min(-1) altitude, 1.0 +/-0.1 l min(-1) and 25 w exercise (sea level, 1.5 +/-0.1 l min(-1) altitude, 1.5 +/- 0.2 l min(-1). For 50 w exercise cardiac output and oxygen delivery were greater at altitude in one subject but were significantly reduced for the remaining five (cardiac output mean difference 3.0 +/- 0.91 min(-1), p = 0.015; DO2 mean difference, 0.56 +/- 0.21 l min(-1) p = 0.028). Acclimatization was therefore adequate to sustain a normal value for oxygen delivery for rest and 25 watts exercise (via compensatory erythropoiesis) but insufficient for 50-watt exercise in five of the six subjects.