Fluorescent microspheres (FM) were used to semi-quantify the effect of orthodontic forces on blood flow in oral tissues in young rats. Forty-five animals had an orthodontic appliance inserted on the first maxillary molar on one side exerting a mesial force of approximately 50 g. Ten animals served as unoperated controls. On days 1, 3, 7, 14, and 21 after the start of the experiment, FM were injected into the left ventricle through an abdominal approach in the experimental and control animals. FM were counted in serial sections from the jaws in the periodontal ligament, pulp, and alveolar bone in a fluorescent microscope. The number of FM per tissue volume and/or tissue weight was taken as a measure of blood flow. The experimental side had significantly lower numbers of FM/mm3 in the periodontal ligament of the first and the second molar on the first day, compared with the contralateral side. However, a steady, significant increase in the number of FM/mm3 in the periodontal and pulpal tissues, and FM/mg in the alveolar bone could be observed on the third and seventh days on the experimental side of the first, second, and third molars compared with the contralateral side, while in the later stages the values of the two sides approached each other. The results of this study indicate that a localized experimental tooth movement initiates a more generalized blood flow response in the periodontal ligament, dental pulp and alveolar bone.