An experimental investigation of the friction factor for fully developed turbulent sodium flow in small smooth tubes is presented. The pressure loss for single and double tubes is measured precisely by means of NaK pressure transducers, under several sets of conditions of fluid flow, temperature, and metallic impurity. From the results obtained, it is found that the friction factor-Reynolds number relationships for the small tubes vary appreciably with the operating time of a sodium loop (1260 to 4100 h). The primary cause of the change in these relations is the deposition of metallic particulates on the tube surfaces.