A series of simulated transient tests was performed to evaluate the effects of heating rates and pressures on the expansion of Zircaloy-4 fuel tube cladding during a period of overheating similar to that which might occur in certain hypothetical coolant failure accidents. The fuel tubes, which were filled with Al2O3 pellets to simulate UO2, were internally pressurized to various pressures. They were inductively heated in a helium atmosphere, so that the temperature increased with time, for 30 sec or until tube failure occurred. Consequently, the range of heating times considered significant for the postulated class of accidents (from a few seconds to 30 sec) was covered, the time for any particular test being determined by the level of pressurization and the applied heating rate. For the test conditions described, maximum swelling occurred at a heating rate and pressure combination that caused perforation in just 30 sec.