Venting and pressure equalization of gas-cooled fast breeder reactor (GCFR) fuel rods can be maintained only as long as axial gas transport paths are available and operating within the fuel pin. Analysis of the chemistry expected within an irradiated GCFR fuel pin shows that axial gas transport paths may become plugged due to the migration of fission product cesium and the formation of low density cesium-urania or cesium-fuel compounds. Cesium transport in the fuel and blanket is controlled by oxygen concentrations and by temperature distribution. Analysis of the shorter Experimental Breeder Reactor II in-reactor fuel pins suggests cesium will be retained in the much longer fast test reactor pins. This will lead to minimal plugging of the gas transport paths.