The effect of fluoride on urinary calculi formation in young rats was investigated. Two studies, in which rats received diets that included either higher calcium (9 g/kg diet) or normal calcium (5 g/kg diet), were conducted At each level of calcium, one group of rats received a high level of fluoride and another a low level of fluoride in the diet. Rats ingesting high fluoride diets exhibited a higher incidence of crystalluria and bladder stones compared with those receiving low fluoride diets. However, compared with higher calcium diets, normal calcium diets delayed the appearance of crystalluria and produced smaller calculi. Calcium and oxalate were the major components of the calculi. Calculi of rats fed the higher calcium and high fluoride diet contained relatively less protein and more calcium compared with calculi formed in rats ingesting the higher calcium and low fluoride diet. The concentration of fluoride in calculi from rats fed high fluoride diets was significantly higher than that of calculi from rats fed low fluoride diets. A significant positive correlation between calcium and fluoride concentration of calculi was observed in rats fed the higher calcium diet only. These studies indicate that ingestion of excess fluoride facilities calcium oxalate crystalluria and promotes the formation of bladder stones in rats, under the experimental conditions used.