The hydrodynamic drag reduction (HDR) in aqueous solutions containing very small amounts of poly(ethylene oxide) (PEO) and polyacrylamide (PAM, 0-100 ppm) was studied under turbulent flow. In this condition, the polymers undergo severe mechanical degradation and loose their capacity to promote drag reduction. The interpretations from a molecular point of view of the mechanical degradation of these flexible polymers under turbulent flow are not consensual. To avoid effects of polymer entanglement and to correlate the mechanical degradation with the intrinsic characteristics of the polymer chain, a polymer concentration lower than 2 ppm was used. For this purpose, a highly accurate rheometer containing a double-gap cell was used to determine the mechanical degradation kinetics. The kinetics was measured directly from the loss of the polymer's capability to promote drag reduction. The comparisons of degradation kinetics for PEO and PAM in aqueous solution allow us to conclude that the stabilities of the two polymers are similar. This new interpretation can be useful to understand the relative mechanical stability of flexible polymers under drag reduction conditions. (C) 2008 Wiley Periodicals, Inc.