The relationship of turbulent drag reduction to molecular weight and molecular weight distribution has been investigated for aqueous polyacrylamide (PAM) solutions of concentration 200, 500 and 1000 ppm. Drag reduction experiments were carried out by circulating solutions in a rectangular flow channel having a flat wall and a wavy wall. In situ measurements of the velocity field were made using particle image velocimetry (PIV). Rheological and rheo-optical measurements carried out over a range of shear rates were used to determine the extent of shear-induced structure formation in the solutions. Molecular weight and molecular weight distribution were analyzed using gel permeation chromatography (GPC). Results showed that diminution of the drag reduction does not always correlate with loss in molecular weight. Long intervals were observed in which the loss of drag reduction occurred at constant molecular weight distribution. In combination with earlier results reported for HPAM solutions, these results support the notion that molecular aggregation could be an important factor in understanding and applying polymer drag reduction. (C) 2004 Elsevier B.V. All rights reserved.