1. Day and night arterial oxygen saturation (Sao2) has been measured in forty-one patients with chronic obstructive pulmonary disease (COPD), mean FEV1 0.84 (range 0.4-1.4)litres, and with a range of daytime Sao2 values of 67-95%. 2. The mean and biggest falls in Sao2 at night were much greater in the patients with lower daytime saturations. However, when falls in arterial oxygen tension (Pao2) were estimated from the decreases in Sao2, there was no correlation between the estimated biggest fall in Pao2 and daytime Sao2 and only a weak correlation between estimated mean fall in Pao2 and daytime Sao2. 3. Measurement of ventilation in four hypoxaemic patients with COPD (range 60-90% Sao2) by respiratory inductance plethysmography showed that nocturnal hypoxaemic dips were accompanied by diminished ventilation, which was not always shown by nasal thermistors. 4. Because nocturnal hypoxaemic dips are transient the ideal alveolar-arterial oxygen difference, which assumes a constant respiratory exchange ratio, cannot be used to assess the mechanism of hypoxaemia. 5. Erythrocyte mass was strongly correlated with daytime Sao2 but this correlation was not significantly improved by including nocturnal hypoxaemia in the regression. 6. The results suggest that greater falls in Sao2 at night are related to lower initial Sao2 values and that the cause may be a reduction in ventilation.