The ultrasonic Doppler flow meter was used to measure digital blood velocity in two groups of ten male volunteers. In the experimental group, digital blood-flow velocity was measured before and after smoking one cigarette. In the control group, two digital blood-flow velocity measurements were made at the same intervals. Blood-flow velocity remained unchanged in the control group. In the experimental group the Doppler shift was reduced by 40±9% during systole, 48±10% during diastole, and the mean Doppler shift was reduced by 42±6% Since the Doppler shift is proportional to blood-flow velocity, it is concluded that smoking a single cigarette reduces mean blood-flow velocity by 42±6% (JAMA 229:1327-1328, 1974)