There are only a few papers in the literature in which parameters related to bone strength are examined over wide range of fluoride concentrations. In this report, chicks were used and fed diets containing fluoride added at 0, 50, 200, or 800 ppm in preparation for measurements on bone strength. The diets were fed for 20 days. After 10 days, the birds were also injected daily with solutions containing either isotonic saline or 5 mg of ethane-1-hydroxy-1,1-diphosphonate (EHDP) as phosphorus/kg body weight or 10 mg of EHDP as phosphorus/kg body weight. The effects of added fluoride and EHDP on bone morphology and the biomechanical properties of bone were then examined. The rate of bone formation was assessed using histological and tetracycline labeling techniques. Administration of high fluoride and/or EHDP resulted in significant changes in bone morphology. The changes effected by the administration of EHDP appeared independent of fluoride induced changes. Biomechanical studies also indicated that both high levels of fluoride and EHDP caused a significant reduction in bone torsional breaking strength. The reductions in breaking strength were independent of changes in body size.