Abstract: This study investigated the potential of Aspartate aminotransferase (AST) as a biological marker to monitor tooth movement by determining its activity in the gingival crevicular fluid (GCF). Six adolescents and seven adults participated in the study. For each subject, an upper first premolar received tipping force (50-75 g) while the opposing premolar served as control. GCF was collected before force application and weekly for 4 weeks. The activity of AST was determined spectrophotometrically (30 °C, 340 nm). AST activity in the GCF of test teeth in all subjects was highest at week 1 and reduced gradually in the next 3 weeks. There was a significant difference in the activity between the test and control teeth in all the subjects throughout the study (p<0.05). There was no significant difference in AST activity between the adult and adolescent subjects (p>0.05). In conclusion, AST appears to have the potential to serve as a biological marker to monitor orthodontic tooth movement.