The strength of osteopenic bone from calcium deprived rats, quail and roosters was significantly reduced after fluoride supplementation. Using a device which measures torque, femurs from rats fed low calcium diets with 100 parts per million fluoride added to the water supply fractured at lower torque values than those values observed for rats fed a low calcium diet without fluoride. Fluoride administration and calcium restriction in adult roosters and young Japanese quail also resulted in a reduction in bone strength. This detrimental effect on bone strength must be considered in any therapeutic attempt to use fluoride ion to stimulate bone formation in osteopenic bone disorders.