The morphology and diameter of the milk vein of 29 healthy Swiss braunvieh cows and the velocity of blood flow through it was measured by colour Doppler ultrasonography before and 10 minutes after they had been sedated with 0.03 mg/kg xylazine. The milk vein was situated immediately under the skin. It had a diameter of 0.8 to 1.6 cm and valves were visible in the milk vein of 12 of the cows. The spectral display appeared as a broad band structure with a wave-like course. The Doppler measurement point was 0.4 to 0.8 cm from the body surface. The diameter of the vein and the blood flow velocity did not differ significantly before and after the cows were sedated. The mean (sd) diameter of the milk vein before they were sedated was 1.3 (0.3) cm. The mean maximum blood flow velocity before they were sedated was 45.4 (12.5) cm/second, the mean velocity was 33.5 (9.5) cm/second, and the mean minimum was 25.8 (11.6) cm/second. There were significant correlations between the blood flow velocities before and after the cows were sedated and between individual cow's blood flow velocities, with correlation coefficients ranging from 0.69 to 0.94.