For over a decade, laser ranging satellites have been used to study variations in the low-order terms of the Earth's gravitational potential. The primary non-tidal source of these variations is redistribution of the mass contained in the Earth's atmosphere, oceans, and continental hydrology. This paper presents an independent detection of these variations using five years of Global Positioning System (GPS) satellite orbits from the International GPS Service. This detection is based on comparisons of observed Earth-referenced positions for the GPS satellites with orbit planes predicted in Earth-centered inertial coordinates. The difference between a GPS UT1-like quantity (UTGPS) and UTC represents the net nodal drift of the GPS constellation and contains both changes in the difference UT1--UTC and effects from even zonal variations in the Earth's gravitational field. Our results indicate that J2 variations at sub-monthly to annual time scales predicted by the National Center for Environmental Prediction's atmosphere model are, in fact, significantly correlated with variations estimated from UTGPS--UT1. In addition, ocean and continental hydrology models suggest that these geophysical fluids make non-negligible contributions at seasonal and other specific time scales. ¿ 2001 American Geophysical Union