Acid and alkaline phosphatase changes have been demonstrated histochemically and biochemically during the early stages of an orthodontic tooth movement cycle. However, quantitative data on neither alveolar bone nor serum phosphatase changes over an entire tooth movement cycle have been reported. This study examined acid, tartrate-resistant acid (TRAP), and alkaline phosphatase changes in serum and alveolar bone during an orthodontic tooth movement cycle in 288 adult male Sprague-Dawley strain rats. The effect of differing initial force magnitudes on phosphatase changes was also examined. Data were obtained from four groups: sham control and three treatment groups (20, 40, and 60 gm activations). Each group (n = 72) was subdivided equally into six sacrifice subgroups (1,3,5,7,10, and 14 days). Treated animals received a precisely loaded orthodontic tipping force to the maxillary molars. Phosphatase measures were obtained by colorimetric assays. Analysis of variance (ANOVA) procedures were performed to examine for differences within groups across time and among groups at each time point. Data from treated animals were combined, adjusted for control values, and examined across time to permit comparisons with previously described histomorphometric changes. Analysis of variance indicated no differences occurred in the control group across time. Significant differences in serum values occurred across time within the force groups (p < 0.01), and among force and control groups at various times in both sera and bone (p < 0.05). In the pooled treatment data, a peak in serum acid phosphatase occurred at day 1 and in bone at day 3 (the later mirroring histomorphometric findings) (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)