A new technique for measuring the vertical component of the ionospheric ambipolar transport velocity Vz in the topside F region is reported. Incoherent backscatter spectra taken on August 7, 9, and 15 (1967) reveal a slight Doppler shift from which the line-of-sight (vertical) velocity of the plasma under observation is calculated. The measurements v/ere taken from 1800 hrs. L.T. to 2200 hrs. L. T. and demonstrate that the velocities during this period were most negative at times corresponding to the rapid cooling of the F region due to ionospheric sunset. The values of Vz range from 40 meters per second downward to 10 meters per second upward.