The effect of fluoride on the physical properties of bone was investigated with the aid of an Instron materials tester. Two groups of weanling female rats, one on an adequate (0.6%) calcium diet and the other on a low (0.1%) diet were given the following dosage levels of fluoride (as NaF) in their deionized drinking water over a fifteen and one-half week period: 3.4 ppm, 10.0 ppm and 45.0 ppm. In the adequate calcium group a significant increase in flexibility in the rat femur was found only at the 45.0 ppm dosage level. This was not offset by a significant decrease in strength. In the low calcium group a similar significant increase in flexibility appeared at the 10.0 ppm dosage level as well as the 45.0 ppm, but a significant decrease in strength at the two dosage levels was observed. These were in direct relation to the amount of fluoride given.