A decrease in the arterial blood saturation by oxygen in patients with POChP is a frequent phenomenon. It is more serious in patients type blue boaters and less frequent among patients type pink puffers. The aim of the paper was to compare the arterial blood saturation by oxygen in the groups examined during two nights: during the first night the patients breathed atmospheric air whereas during the second night they were given oxygen. The author also studied the influence of oxygenation of an organism on the frequency of cardiac rhythm disorders (ZRS). The author examined a group of 20 patients with the predominance of chronic bronchitis--blue boaters (average VC was 1.95 l, FEV1--0.81 l, PaO2 while breathing atmospheric air 52 mm Hg and 68 mm Hg after giving oxygen, PaCO2 47 and 51 mm Hg respectively) and 20 patients with the predominance of emphysema--pink puffers (average VC--2.30 l, FEV1--0.86 l, PaO2 while breathing atmospheric air 60 mm Hg and 70 mm Hg after giving oxygen, PaCO2 39 and 40 mm respectively). It was found that the patients with heavy hypoxaemia and hypercapnia had worse arterial blood saturation by oxygen during the two nights of investigation in comparison with the other group. The author also found more frequent cardiac rhythm disorders in this group of patients. Giving oxygen improved blood oxygenation in the two groups and lowered the frequency of cardiac rhythm disorders. The results obtained indicate to the need of oxygen therapy in patients with advanced POChP, especially during the night so as to avoid nocturnal hypoxaemia of an organism.