Direct electrical heating (DEH) experiments have been analyzed to study the fuel and fission gas response to simulated thermal transient conditions in unirradiated UO2 and irradiated mixed-oxide fuels. The results indicate that both fission gas and fuel melting strongly influence the fuel response behavior during transients. The information obtained from the DEH tests has been used to draw a preliminary “fuel response diagram” that contributes to an understanding of such macroscopic fuel behavior as gross fuel swelling and fuel failure as functions of the transient time and heating rate. A comparison of these test results and the trend depicted by the fuel response diagram with results obtained from experiments performed in the transient reactor test facility indicates that certain aspects of fuel behavior during transients can be reasonably simulated by the DEH technique.