This study compared the ventilatory response to 20 min of acute isocapnic hypoxia (end-tidal PO2, 50 mmHg) using the technique of dynamic end-tidal forcing in young (Y) and old (O) men. Two groups of non-smoking male subjects (mean ±plusmn; s.d. age: Y, 29.8 ±plusmn; 6.9 years; O, 73.4 ± 2.8 years) with similar body size, normal age-predicted spirometry, and normal moderate levels of physical activity were studied. Compared with baseline ventilation in euoxia (10.79 ± 1.99 and 11.88 ± 0.91 l min-1) both groups responded to the abrupt onset of isocapnic hypoxia with peak ventilatory responses of 22.58 ± 2.60 and 24.56 ± 2.54 l min-1 for Y and O, respectively (not significant, n.s.). Both groups demonstrated a significant increment in neuromuscular drive (i.e. tidal volume (VT)/inspiratory time (TI); 0.46 ± 0.06 to 0.91 ± 0.15 and 0.48 ± 0.06 to 0.91 ± 0.12 l s-1 for Y and O, respectively) with a small (but also significant) change in central timing (TI/total ventilation time (Ttot); 0.38 ± 0.02 to 0.41 ± 0.02 and 0.42 ± 0.02 to 0.45 ± 0.02 for Y and O, respectively). Oxygen sensitivity was assessed using Weil's equation, and gave a hyperbolic factor (A) of 282 ± 75 and 317 ± 72, and using the linear equation: change in expiratory minute volume (ΔVE)/change in arterial O2 saturation (ΔSa,O2) which gave -1.17 ± 0.57 and -1.17 ± 0.42 l min-1 %-1 (n.s.) for Y and O, respectively. After 20 min of sustained isocapnic hypoxia, ventilation declined to 14.29 ± 1.92 and 16.85 ±plusmn; 2.34 l min-1 for Y and O, respectively (n.s.). The acute response to hypoxia was characterised by similar time constants (16.0 ±plusmn; 5.4 and 18.5 ±plusmn; 6.7 s) and time delays (4.8 ±plusmn; 2.1 and 4.6 ±plusmn; 1.9 s) for Y and O, respectively. Thus, the dynamic ventilatory response to acute isocapnic hypoxia is maintained into the eighth decade in a group of habitually active elderly men. Experimental Physiology (2001) 86.1, 117-126.