Plasma drift measurements for the F region of the ionosphere have been made by the Arecibo incoherent scatter radar over the course of more than one solar cycle. The analysis of the data indicates that there are significant differences in the nighttime drifts between solar minimum and solar maximum. In general, these results agree qualitatively with results obtained through previous studies which have been done for incoherent scatter measurements from Jicamarca, Peru, as well as with some characteristics of model simulations. ¿ American Geophysical Union 1990