The effect of a low dose of glucocorticoids on dentin formation and progression of dental caries was studied in young rats. The animals were inoculated in the mouth with Streptococcus sobrinus and fed ad libitum a high-sucrose, a high-starch, or a reference diet. The cortisone or placebo pellet was implanted subcutaneously in the back of the necks of the animals. After a 6-week experimental period the areas of dentin formation and dentinal caries were quantified planimetrically. Schiff's staining was used to classify caries. Cortisone reduced the dentin formation both in high-starch and reference diets, but it did not further affect the reducing effect of the high-sucrose diet on dentin formation. Cortisone did not affect the number or severity of dentinal carious lesions.