A main factor which might cause cardiac arrhythmias and arterial hypotension during thoracic operations is surgical manipulation around the pericardium and the lung hilus. Halothane and enflurane were compared as to the occurrence of arrhythmias and hypotension caused by standardized surgical stimulation. Twenty-eight patients scheduled for thoracotomy were studied. The patients were selected in randomized order, and the anaesthetic agent in use was unknown to the surgeon. ECG, intra-arterial blood pressure and central venous pressure were recorded continuously. Six patients in the halothane group (n = 14) and nine patients in the enflurane group (n = 14) developed premature atrial contractions and nodal rhythm, while nine patients in the halothane group and five in the enflurane group developed a fall in systolic blood pressure of more than 20 mmHg (2.7 kPa). No statistically significant difference in the occurrence of cardiac arrhythmias and hypotension was found between the two anaesthetic groups.