Fluorescent microspheres were used to visualize and semi-quantify blood flow in the periodontal ligament and dental pulp during experimental mesial movement of the first maxillary molar in rats. The orthodontic appliance consisted of a coil spring connecting the first maxillary molar on one side to the central incisors. After 5 days with a continuous force the animals were reanaesthetized and a catheter was placed in the left ventricle for injection of fluorescent microspheres. The fluorescent microspheres were counted on serial sections from the jaws on the mesial and distal aspects of the periodontal ligament and pulp of the molar teeth, in a fluorescent microscope. Blood flow was related to the number of fluorescent microspheres in the tissues under investigation and in a reference blood sample. There was a substantial increase in blood flow in all areas examined on the experimental side compared to the control side.