During haemodialysis (HD), allowing important CO2- unloading, an irregular breathing pattern (BP) is frequently observed. This has been attributed to a decrease in central chemoreceptor firing, with a greater contribution of the peripheral chemoreceptors in the chemical drive to breathe. To provide further evidence for these findings we studied five patients with end-stage renal failure in chronic HD. They underwent HD with a cuprophane membrane and acetate-containing dialysate. Ventilation was measured continuously using respiratory inductance plethysmography. Oxygen was administered for 30 min, using nasal cannulae, at a rate of 6 l.min-1, starting 130 min after the onset of the HD. Blood gases were taken from the arterial line. During the initial air breathing, arterial oxygen tension (PaO2) decreased from 12.3 +/- 1.2 kPa (92.8 +/- 8.9 mmHg) at 0 min to 10.5 +/- 1.8 kPa (79.0 +/- 13.3 mmHg) at 2 h (p less than 0.01) (mean +/- SD). All patients showed irregular breathing with 1.4 +/- 0.6 apnoeas exceeding 10 s per 10 min after 2 h. Minute ventilation decreased from 6.8 +/- 1.9 l.min-1 at 0 min to 5.4 +/- 1.3 l.min-1 at 2 h (p less than 0.05). During the O2 breathing, PaO2 increased to 26.3 +/- 4.0 kPa (197.8 +/- 30.3 mmHg) (p less than 0.001), while arterial carbon dioxide tension (PaCO2) remained unchanged. The irregular BP previously observed vanished completely. The mean number of apnoeas exceeding 10 s per 10 min decreased to 0.08 +/- 0.12 during O2 (p less than 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)