Aggrecan is the self-assembling proteoglycan complex whose physiological function is to provide a hydrated gel in cartilage that stabilizes the spatial distribution of the collagen fibers, and absorbs and resists compressive loads. The linear and nonlinear viscoelastic behavior of aggrecan solutions was studied as a function of aggrecan concentration and ionic strength. At physiological ionic strength, I=0.15, a sol-to-gel transition occurs at an aggrecan concentration just above the overlap value. Concentrated solutions exhibit a reversible yield point similar to that predicted and observed for close-packed dispersions of soft spheres. In contrast to the behavior of linear polyelectrolyte solutions, the storage modulus increases with ionic strength, until I=0.75, above which the modulus decreases. In light of the dense polyelectrolyte brush structure of aggrecan, we suggest that this behavior is a manifestation of the crossover from “osmotic brush” to “salted brush” conditions.