Late postoperative episodic and constant hypoxaemia and associated ECG abnormalities. Twenty-two patients without cardiopulmonary disease and undergoing elective major abdominal surgery were monitored continuously with a Holter tape recorder and a pulse oximeter on one night before operation and the first two nights after operation (23:00 to 07:00), without oxygen therapy. Mean heart rate increased 16 beat min-1 (P less than 0.001) and mean oxygen saturation (SaO2) decreased 2.6% (P less than 0.001) after operation. Episodic oxygen desaturation to less than 80% occurred in four patients before operation, but in 13 patients after operation (P less than 0.05). ECG abnormalities were observed in 10 patients before operation and in 16 patients after operation (ns). Individual maximum ST depression was more pronounced after than before operation (P less than 0.05). Episodic desaturation was related closely to tachycardia in six patients before operation and one patient after operation; before operation to atrioventricular block in one patient, and after operation to ST depression in two patients. Mean SaO2 on the second night after operation correlated with total dose of opioid for pain relief (rs = -0.48; P less than 0.05), and postoperative decrease in SaO2 correlated with postoperative increase in heart rate (rs = -0.43; P less than 0.05). No patient had postoperative cardiac complications indicating treatment. 