Data from a worldwide Global Positioning System (GPS) tracking experiment have been used to determine variations in Earth rotation (UT1-UTC) over a time period of three weeks. Kalman filtering and smoothing enabled changes in UT1-UTC over intervals of 2 to 24 hrs to be detected with the GPS data. Internal consistency checks and comparisons with other solutions from very long baseline interferometry (VLBI) and satellite laser ranging (SLR) indicate that the GPS UT1-UTC estimates are accurate to about 2 cm. Comparison of GPS-estimated variations in UT1-UTC with 2-hr time resolution over 4 days with predicted variations computed from diurnal and semi-diurnal oceanic tidal contributions strongly suggests that the observed periodic sub-daily variations of ~0.1 msec (5 cm) are largely of tidal origin. ¿ American Geophysical Union 1992