Fluid displacement patterns were observed in two- and three-dimensional (2- and 3-D), gravity-driven drainage experiments. A nonintrusive, laser-induced fluorescence technique, using refractive index matching between solid and fluid phases and subsequent imaging processing, enabled measurement of the flow patterns in porous media. The methodology allows visualization of 3-D pore-scale flow dynamics of two immiscible fluid phases in porous media at a low capillary number (Ca ≈ 10-4). Qualitative analyses indicate considerable diversity in the 2-D and 3-D displacement patterns of the invading fluid within and among the experimental systems. Quantitative analyses of these patterns, however, provide unique functional forms that characterize the invasion behavior of the nonwetting fluids, with clear differences in behavior between 2-D and 3-D systems. In particular, scaling relations are defined which describe the maximum vertical advance, volumetric fraction, total surface area (or, equivalently, the ratio of total surface area to volume), and specific surface area of the invading fluid. These findings are in excellent agreement with predictions from dynamic invasion percolation growth models, which were developed originally without consideration of buoyancy effects.