A series of drilling fluid invasion experiments were conducted on Berea sandstone cores to measure the resistivity profile characteristics of the invaded zone. A specially designed dynamic filtration core holder was used in this study. Resistivity measuring electrodes allowed the measurement of resistivities and permeabilities for 4 sections of the core while circulating the mud. Six kinds of muds with different compositions and different dynamic filtration rates were circulated across the face of the core for 10 hours. For a brine saturated core when Rmf>> Rw , the resistivity profile along the core shows a wide transition zone. Only the first inch or two of the core was completely flushed. The transition zone is much larger than that obtained with a miscible displacement at constant rate. Since invasion rate decreases by a factor of 10 over the course of the experiment as the mud cake builds up, the longitudinal dispersion coefficient and Peclet number decrease. This makes the resistivity profile non-uniform and lengthens the transition zone. Molecular diffusion has little effect on the resistivity profile and is much smaller than convective effects, even at Darcy velocities as low as 0. 043 cm/h. This effect was observed in a sequence of static and dynamic filtration experiments. However, after the invasion of mud filtrate ceases, molecular diffusion makes the resistivity along the core uniform over a time period that is inversely related to the permeability of the core. For an oil saturated core a resistivity annulus and transition zone were clearly seen when the resistivity of the mud filtrate is much larger than that of the irreducible brine in the core. The resistivity annulus becomes wider with time and with distance from the face of the core exposed to the mud. The width of the annulus and its rate of propagation and growth depend on the properties of the mud and the permeability of the core. When Rmf and Rw are of the same magnitude no resistivity annulus is observed. The resistivity profile along the core quickly becomes uniform.