Adult rabbits were subjected to running on a treadmill three times daily for 1, 5, and 30 days. The animals were injected with 35S-sulfate and killed 24 h later. The concentration of free and bound glycosaminoglycans (GAG) in plasma, as well as the specific radioactivities of the fractions, was determined. Thirty days of running produced a statistically significant increase in the concentrations of both free and bound plasma GAG, whereas only the latter fraction was increased after 5 days of running. Consistent changes in the specific radioactivities of the fractions were not discernible.