Acid Glycosaminoglycans (GAGs) were measured, in terms of hexuronic acid, following alkaline treatment and Ecteola chromatography, in plasma obtained from healthy volunteers, blood-donors, amateur soccer players and university students, and from hospitalized subjects at the end of their convalescence. Diurnal variations of plasma GAG concentration, with significant decrease during the morning, were obtained in students and patients, suggesting hormonal influences. Furthermore, moderate modifications of plasma GAG concentration were observed in students following cyclo ergometer exercise which were consistent in each subject with cortisol mediated changes. However, the absolute value of plasma GAG concentration appears to be depending on the physical training of the subject, being significantly higher in the soccer players and in the blood-donors than in the other groups of subjects, chiefly composed of sedentary individuals. The intramuscular connective tissue is then suggested to represent a main site of origin of plasma GAGs.