The elastic and anelastic parameters of sandstones at ultrasonic frequencies under low uniaxial pressure are strongly influenced by the fluid content. The two main mechanisms controlling the rock behavior are based on (1) local fluid flow and (2) a grain surface effect which results in a modulus reduction. Both mechanisms depend fundamentally on the microstructure, the critical factor controlling seismic wave velocity and attenuation in rocks under low pressure. Ultrasonic measurements have been performed on a fluid-containing sandstone under hydrostatic pressure up to 200 MPa to investigate the mechanisms affecting rock behavior under in situ pressure conditions. At high hydrostatic pressure the measured P wave velocities indicate a change in the rock microstructure and in the influence of the fluid. By means of two models the effects of the mechanisms of local fluid flow <Murphy et al., 1986> and global fluid flow <Biot, 1956a, b> were calculated, and the loss mechanisms were compared in their capability of explaining the experimental data. The experimental and theoretical results show that for the investigated fluid-containing sandstone both the local fluid flow mechanism and, in the case of polar fluids, the modulus reduction are still effective at high external pressure.¿ 1997 American Geophysical Union