The Argonne Mark-II loop in the core of the TREAT reactor is used to investigate the thermodynamics of a sodium-cooled fast reactor fuel pin. This experiment on the top 30.44 cm of an unirradiated fast test reactor (FTR) fuel pin was the first in a series to be conducted in support of the liquid metal fast breeder reactor (LMFBR) program and as such constituted an exploration into the ability of the loop and reactor facility to produce simulations of a wide range of flow conditions in assemblies of sodium-cooled fast reactor fuel pins. The objective of this first experiment (L1) was to approach but not cross over the threshold of the structural integrity of the cladding by reducing the sodium velocity while the pin was continuing to generate heat at the full power +20% rate of an FTR pin. This objective was achieved despite perturbations in sodium velocity and temperature of greater amplitude and frequency than anticipated and with some irreversible structural changes in the pin.