The ventilatory equivalent for CO2 defines ventilatory efficiency largely independent of metabolism. An impairment of ventilatory efficiency may be caused by an increase in either anatomical or physiological dead space, the latter being the most important mechanism in the hyperpnoea of heart failure, pulmonary embolism, pulmonary hypertension and the former in restrictive lung disease. However, normal values for ventilatory efficiency have not yet been established. We investigated 101 (56 men) healthy volunteers, aged 16-75 years, measuring ventilation and gas exchange at rest (n = 64) and on exercise (modified Naughton protocol, n = 101). Age and sex dependent normal values for ventilatory efficiency at rest defined as the ratio ventilation:carbon dioxide output (VE:VCO2), exercise ventilatory efficiency during exercise, defined as the slope of the linear relationship between ventilation and carbon dioxide output (VE vs VCO2 slope), oxygen uptake at the anaerobic threshold and at maximum (VO2AT, VO2max, respectively) and breathing reserve were established. Ventilatory efficiency at rest was largely independent of age, but was smaller in the men than in the women [VE:VCO2 50.5 (SD 8.8) vs 57.6 (SD 12.6) P < 0.05]. Ventilatory efficiency during exercise declined significantly with age and was smaller in the men than in the women (men: (VE vs VCO2 slope = 0.13 x age + 19.9; women: VE vs VCO2 slope = 0.12 x age + 24.4). The VO2AT and VO2max were 23 (SD 5) and 39 (SD 7) ml O2 x kg x min(-1) in the men and 18 (SD 4) and 32 (SD 7) in the women, respectively, and declined significantly with age. The VO2AT was reached at 58 (SD 9)% VO2max. Breathing reserve at the end of exercise was 41% and was independent of sex and age. It was concluded from this study that ventilatory efficiency as well as peak oxygen uptake are age and sex dependent in adults.