Experiments were performed with a 19-rod test assembly in the fuel failure mockup sodium loop in which fuel rods were simulated by electrical cartridge heaters having the same external configuration, spacer arrangement, temperature, and heat flux as those of a typical liquid-metal fast breeder reactor (LMFBR). Temperatures were measured within the rod bundle, at the exit, and along the duct walls of the rod assembly for widely varying conditions of flow and power density and for nonuniform radial power distribution. Significant differences in temperature were measured around the duct periphery. These appeared to be linearly dependent on the power density. Flow reduction caused a decrease in these measured temperature differences. Temperatures at the exit of interior subchannels agreed with analytical predictions. Those of peripheral channels, however, indicated the existence of significant swirl flow around the rod bundle and required a separate analytical treatment. In situ radiographs indicated distortion of the rod bundle toward the duct wall at elevations where the spiral wire-wrap spacers did not touch the duct wall. At each elevation, the measured circumferential temperature profile was related to the position of the wire-wrap spacers relative to the duct wall. Higher temperatures were measured on duct walls not in contact with the spacers. The measured differences in temperature around the duct periphery were of sufficient magnitude that, if present in LMFBR cores, their effect should be considered in evaluating temperature-dependent material growth due to fast-neutron irradiation.