The purpose of this study was to clarify the histologic changes in the expansion of midpalatal suture by the tensile forces. 39 Sprague-Dawley rats were divided into a control group (3 rats) and three experimental groups (36 rats)-group 1, pressured with a light force (50-75g), group 2, with a heavy force (250-300g) and group 3, with a heavy force (250-300g) plus laser irradiation. Autoradiographic and histopathologic observations were performed in 12, 24, 48 and 96 hours after force delivery. The results were as follows: 1. The anterior portion of midpalatal suture was more separated than the posterior portion in all experimental groups. Group 2 showed more separation than group 1 and no difference to group 3. 2. Ligament tearing appeared intensively in 24 hours, but the sutural matrices increased with times.: Group 2 showed more tearing than group 1, and active regeneration of sutural matrices was observed in group 3. 3. Vascular dilatation appeared intensively in 24 hours and decreased with times. : The anterior portion of midpalatal suture showed more dilatation than the posterior portion.: The changes was the greatest in group 3, group 2, group 1, in that order. 4. New bone formation and the new capillary prolieferation began to appear in 12 hours and increased with times.; Group 2 showed more changes than group 1 and no difference to group 3. 5. Infiltration of inflammatory cells was little observed and was the greatest in group 2, group 1, group 3, in that order. 6. Positive reaction of cells to [^³H] thymidine was the greatest in 24 hours, and decreased with times: The reaction was the greatest in group 3, group 2, group 1, in that order.