Normal stress measurements were made on dilute solutions of polyisobutylene in three low‐viscosity solvents at high shear rates with a jet thrust apparatus in which the thrust tube was mounted on the frame of an analytical balance. Thrust measurements could be easily read to 0.01 g and replicate measurements agreed closely with the original ones. Mechanical degradation of polyisobutylene in cyclohexane and toluene caused significant reductions in(P11−P22)wvalues at high shear rates and in the amount of drag reduction in turbulent flow. Molecular weight distribution curves and estimates of viscosity‐average molecular weights of the fresh and degraded solutions in toluene indicate that these effects noted in this solvent are caused by the breakdown of small amounts of high molecular weight polymer, with only a small change in the viscosity‐average molecular weight.