A method for automatically placing a range gate over a moving blood vessel during ultrasound imaging. Doppler data received from a number of depths in the tissue is analyzed in order to calculate the average velocity of the tissue at each depth. A search is performed in the average velocities to select a maximum velocity. The maximum velocity is associated with a blood vessel and a range gate is placed at a depth corresponding to the maximum velocity. In a presently preferred embodiment of the invention, the average velocity is calculated by performing a first lag autocorrelation of the echo data received from each depth in response to a series of Doppler pulses.