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 ± s.d. age: Y, 29.8 ± 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 ± 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 ± 5.4 and 18.5 ± 6.7 s) and time delays (4.8 ± 2.1 and 4.6 ± 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.