Hypoxaemia and breathing irregularities have been shown to occur during haemodialysis in patients with chronic renal failure. This study examined the role of hypoxia in the genesis of the irregular breathing during haemodialysis. The ventilatory patterns using respiratory inductance plethysmography and arterial blood gases were studied in seven males with chronic renal failure on long-term haemodialysis. The study was carried out before and during dialysis on one day without (D1) and another day with intranasal oxygen at 4 L x min(-1) (D2). On D1, mean (SD) arterial oxygen tension (Pa,O2) fell 1.9 (0.9) kPa (p<0.001) and mean minute ventilation (V'E) fell 1.9 (1.1) L x min(-1) (p<0.01) during dialysis. The arterial carbon dioxide tension (Pa,CO2) did not show a significant decrease (4.7 (0.2) kPa before and 4.6 (0.2) kPa during dialysis). Cumulative number of apnoeas was 64 and the coefficients of variation (COV) of respiratory frequency (fR) and tidal volume (VT) were 29.6 (11.9) and 38.2 (11.9)%, respectively. On D2, mean Pa,O2 remained stable (20.4 (4.1) kPa before, 21.3 (4.1) kPa during dialysis). There was no significant change in mean V'E (6.4 (0.9) L x min(-1) before, 5.5 (0.5) L x min(-1) during dialysis). Pa,CO2 decrease was not significant but the fall was greater (4.8 (0.1) kPa before, 45 (0.5) kPa during dialysis). Cumulative number of apnoeas was 94 and the COVs offR and VT were 35.8 (5.1) and 40.5 (11.3)%, respectively. Oxygen administration did not significantly affect the haemodialysis-induced changes in ventilation and breathing pattern, despite a significant protective effect from the fall in arterial oxygen tension. It was concluded that the fall in arterial oxygen tension is not the main determinant of breathing irregularities during haemodialysis.