Monthly mean gravitational field parameters (denoted here as Ceven) that represent linear combinations of the primarily even-degree zonal spherical harmonic coefficients of the Earth's gravitational field have been recovered using LAGEOS I data and are compared with those derived from gridded global surface pressure data of the National Meteorological Center (NMC) spanning 1984--1992. The effect of equilibrium ocean tides and surface water variations are also considered. Atmospheric pressure and surface water fluctuations are shown to be the dominant causes of the observed annual Ceven variations. Closure with observations is seen at the 1&sgr; level when atmospheric pressure, ocean tide and surface water effects are included. Equilibrium ocean tides are shown to be the main source of excitation at the semiannual period with closure at the 1&sgr; level seen when both atmospheric pressure and ocean tide effects are included. The inverted barometer (IB) case is shown to give the best agreement with the observation series. The potential of the observed Ceven variations for monitoring mass variations in the polar regions of the Earth and the effect of the land-ocean mask in the IB calculation are discussed. ¿ American Geophysical Union 1996