These experiments were conducted to investigate the effects of high oral intakes of ascorbic acid on bone metabolism in young guinea pigs and to inquire into the mechanism mediating these effects. Male guinea pigs 14 days old fed 8.7% of ascorbic acid for 6 weeks had decreased bone density and decreased urinary hydroxyproline compared to control animals fed 0.2% of ascorbic acid. To assess whether these changes were due to the acidity of the diet or to vitamin activity, salts of ascorbic acid, salts of erythorbic acid and free erythorbic acid were fed. There were no significant bone changes in any of these groups. The results strongly indicate that the acidity of ascorbic acid plays a part in the decrease in bone density but do not rule out that some other metabolic property specific to ascorbic acid may also be implicated.