The prevalence of central apnoea and periodic breathing is increased in the elderly. This implies that the chemical control of breathing might become less stable with ageing. To investigate this, we measured loop gain in healthy elderly individuals using proportional assist ventilation. Loop gain is an engineering term that describes the stability of a system controlled by feedback loops, such as the respiratory control system. A loop gain close to zero indicates a stable system, whereas a loop gain close to or greater than one indicates an unstable system. Eleven healthy elderly subjects were studied with a mean +/- S.D. age and body mass index (BMI) of 71 +/- 5 years and 25 +/- 3 kg m(-2), respectively. We also studied a small group of elderly individuals with obstructive sleep apnoea (OSA) for comparison (n = 3, age 68 +/- 1 years, BMI 32 +/- 11 kg m(-2)). Comparisons were made with previously studied young individuals (age 27 +/- 4 years, BMI 23 +/- 1 kg m(-2)). We found significantly lower loop gains in the healthy elderly group (loop gain <or= 0.25) compared with the young group (loop gain <or= 0.47, P = 0.001). Also, we found quite low loop gains in the elderly OSA group (loop gain <or= 0.26). We conclude that the chemical control of breathing does not become unstable with ageing and is thus an unlikely cause of central (and possibly obstructive) apnoeas in this population.