Chronic smoking, particularly in hypertensives, is associated with an increased frequency of cerebral infarction and subarachnoid haemorrhage; cerebral blood flow (CBF) is also decreased in chronic smokers. The acute effects of smoking upon CBF are less clear. The present study in six volunteers investigated the immediate effects of smoking (and inhaling) three cigarettes, each separated by a 2 hr gap, upon heart rate, blood pressure (BP) and CBF assessed by xenon inhalation and flow velocities in the middle cerebral artery by transcranial Doppler ultrasound. Smoking caused significant increases in heart rate, BP and middle cerebral artery flow velocities, and a significant fall in CBF (individual falls in hemispherical flow could be as great as 40%). These haemodynamic effects are probably caused by nicotine-induced changes in catecholamine release. The possible clinical significance of the BP and CBF changes induced by acute smoking in high-risk groups is discussed.