Male weanling rats were supplied a standard mixed diet (SIFF Norsk Standard) and were given tap water containing 0.02 ppm of F (control group) or water containing 45 ppm of F (fluoridated group) for 4 weeks. At this time the right hind leg of all animals was paralysed by severing the sciatic and femoral nerve. The rats were then continued on the same food and water regimen for 3 weeks. The specific gravity, ash weight and calcium content of the dry, defatted femora were measured. The differences in ash and calcium content in the paralysed and the normal legs were found to be similar in both groups. In the fluoridated animals, however, the normal limbs showed significantly higher mean values of ash content and specific gravity than the corresponding legs of the control animals. The mean ash content of the paralysed limbs of the fluoridated animals was slightly higher, and the mean calcium content and specific gravity were similar to the values observed in the normal limbs of the control animals. Apparently, the high fluoride intake was associated with an increased rate of bone formation, eliminating the effects of disuse atrophy of bone in the paralysed femora.