The time dependent drag reducing ability of a surfactant solution was investigated experimentally from the viewpoint of developing the surfactant solution as a drag reducing fluid in district heating systems. Aqueous solutions of a cationic surfactant were subjected to turbulent pipe flows continuously to induce degradation. The results show that surfactant solutions lose their drag reducing ability abruptly after a stable period. In this study, this phenomenon is named 'break-down' of drag reducing ability. In the broken-down solution large aggregates of surfactant and fragments of surfactant molecules were found. The loss of birefringence was also observed under high shear-flow. The break-down phenomenon appears to be caused by the lowering of effective micelle concentration caused by the precipitation of surfactant molecules and chemical reactions of surfactant molecules with dissolved oxygen at high temperature.