Glycosaminoglycans (GAG) in gingival crevicular fluid (GCF) were investigated by cellulose acetate electrophoresis of simultaneously collected samples from the mesial and distal surfaces of teeth in 3 groups of young persons. In a control group, which had not undergone orthodontic treatment, a major band of hyaluronic acid (HA) and a minor band of chondroitin sulphate (CS) were present. No differences in the mean content of either GAG between the mesial and distal surfaces were detected. From teeth undergoing movement by fixed appliances (active group), a raised mean level of CS was present in GCF from the surface towards which movement was directed. Teeth held passively by an appliance following cessation of active movement (retention group) showed raised levels of CS at mesial and distal surfaces. A heparan sulphate-like GAG was commonly present in this group only. No significant increase in the levels of HA were detected at the mesial and distal surfaces of either the active or the retention groups, despite increased GCF flow rates unassociated with more severe gingival inflammation. The GAG composition of GCF, particularly CS, appears to reflect changes occurring in the deeper periodontal tissues of alveolar bone and periodontal ligament during orthodontic treatment.