A model employing Darcy's law has been developed to describe the transient pressure field within interconnected porosity of mixed-oxide liquid-metal fast breeder reactor fuel during hypothetical reactor accidents. Pressure increases are due both to fission gas released from fuel grains and fill gas originally present within fuel pores. Calculations utilizing the model have been performed for an out-of-pile test prior to fuel melting with both clad and unclad conditions being treated. Redistribution of gas from the source region in the relatively high-porosity unrestructured fuel to a low-porosity restructured fuel was shown to exist in all cases considered. Even for the unclad case, significant internal pressurization was predicted by the model, which could prove important in subsequent fuel breakp and motion.