HF Doppler observations of vertical plasma drifts in the equatorial F region at Trivandrum (dip: 0.9¿S), conducted during 1984--1986, are presented. The observations reveal a fairly consistent diurnal variation pattern with the drift velocities directing upward during day and downward during night. The diurnal pattern is dominated by the characteristic prereversal enhancement that exhibits the most striking seasonal, solar, and magnetic activity effects. Under high solar activity, the average peak velocity of the enhancement is found to drop from ~40 m s-1 in equinox to ~10 m s-1 in summer. Likewise, the average equinoctial peak velocity drops almost by a factor of 3 as the solar activity index (S10.7) falls from 125 to 70 units. The peak velocity has a unique magnetic activity dependence; it drops by more than a factor of 2 as the activity changes from quiet to moderate but assumes well above the quiet day value for high activity. The corresponding variations for the other times of the day are nowhere as pronounced as for the period of the prereversal enhancement. The observations presented here are found to be in broad agreement with those reported for Jicamarca and are also in accordance with the model predictions based on the F region dynamo. ¿ American Geophysical Union 1989