OBJECTIVE This study was designed to examine the changes of aspartate aminotransferase (AST) and alkaline phosphatase(ALP) activities in gingival crevicular fluid (GCF) during rapid palatal expansion, and to explore the relationship between these changes and periodontal tissue remodeling. METHODS Thirty-eight patients (aged 10.5 to 12.8years) who required opening of the palatal suture as the first stage of their orthodontic treatment were included and studied. The patients were divided into two groups: control group (18 cases) and experimental group (20 cases). GCF samples were collected at 6 observation periods with filter paper strips: before activation, 24 hours and 7 days after activation, 7 days, 14 days, and 28 days in retention. AST and ALP activities were determined by a full automatic biochemical analyzer, and the results were expressed as total AST and ALP activities. The data was analyzed with SAS (r) Proprietary Software Version 9.00 for paired t test and two-sample t test. RESULTS AST activity was significantly elevated after 24-hour activation in the test group (P<0.05), but after 7-day activation in the control group (P<0.01). Both in the control and experimental group, AST activity remained high until 28-day in retention (P<0.01). AST activity in the test group was significantly greater compared to the control group from 24-hour activation to 28-day in retention (P<0.05). ALP activity increased significantly from 7-day activation to 28-day retention in the control and experimental group (P<0.01), and there was statistically significant difference between the control and the experimental group from 7-day to 28-day in retention (P<0.01). CONCLUSIONS Our results suggest that AST and ALP levels in GCF may reflect the tissue remodeling which occurs in the periodontium during rapid palatal expansion to some degree.