Two groups of male subjects, mean age 25 and 50 years, were studied at rest supine during air breathing, 15-11 and 100% Oa breathing, sitting at rest and during exercise up to maximal work load, and supine during a constant submaximal work load combined with D L~~ measurements. At rest supine, the mean and range of PaoI was 90.2 (79-97) in the young and 78.2 (68-88) mmHg in the middle-aged. PA-^^^ was 4.3 (-1-14) and 23.4 (15-33) mmHg, respectively. Corresponding SaOp values were 97.3 (95.7-99.0) and 94.5 (91.7-98.6) per cent. During hypoxia, the Pao, and PA-aOl values were lower in the young than the middle-aged. During hyperoxia the PaOI and PA-aol were 611 (574-644) and 48(31-60) in the young and 573 (492-612) and 93 (60-108) mmHg in the middle-aged, respectively. VD and VD/VT were constantly higher in the middle-aged. At max. work load, no great difference in Paoz (87.5 and 85.8 mmHg) or SaOI (95.8 and 94.2 %) was found. The mean D L~~ values were 39.2 (young) and 36.8 (middleaged) ml/mmHg.min, as measured at VO, 1490 and 1 656 ml/min, and at VE 35.9 and 43.6 Ijmin, respectively.