Sodium voiding during the initial time period subsequent to boiling inception was studied experimentally under forced convection with system parameters typical of liquid-metal fast breeder reactors. The annular flow area of the test section simulated a single reactor fuel element of 3-ft heated length. Transient void formations were measured along the test section length. Initial sodium-voiding characteristics were related to the maximum bulk superheat existing in the test section at the time of boiling inception. Significant differences in test section voiding were obtained under conditions of zero and high (100 to 160°F) superheats. Loop and pot-type reactor simulation conditions were employed.