The phenomenon of shear-induced structure formation has been investigated for several drag reducing polymers. Aqueous solutions of high molecular weight partially hydrolyzed polyacrylamide, polyacrylamide, and poly(ethylene oxide) have been examined in the concentration range of 100-4000 ppm by weight. Experimental protocols included rheological and rheo-optical characterization under simple shear flow, and molecular weight analysis by chromatography following various shear histories. All solutions exhibited time-dependent stresses and turbidity patterns under high shear, indicative of structure formation. Two regimes of behavior were found with the partially hydrolyzed polyacrylamide. At concentrations below 1500 ppm, aggregate formation occurred above a critical shear rate on time scales of minutes that relaxed back over similar time scales to an apparent homogenous state on cessation of flow. However, these sheared solutions developed irreversible, large-scale turbidity and eventual precipitation on quiescent storage over several days. At higher concentrations, phase-separated droplets formed under shear that also showed ripening of the turbidity and eventual precipitation upon quiescent storage for days. The kinetics of the semi-reversible aggregate formation and relaxation processes in the lower concentration regime showed a regular variation with shear rate and concentration. The strength of the structure formation and precipitation processes was lessened in the presence of salts. Similar behavior patterns were exhibited by the other systems, however, the strength of the structure formation was also somewhat lessened compared to the partially hydrolyzed polyacrylamide. Correlations with solutions being used in ongoing turbulent drag reduction studies suggest that such structure formation may play an important role in the drag reduction process. (C) 2003 Elsevier B.V. All rights reserved.