The effect of fluoride in drinking water on the progression of dentinal caries and dentin apposition was studied separately in young and old Wistar rats. The animals were inoculated withStreptococcus sobrinusand fed ad libitum on a 43% sucrose diet and distilled water supplemented with 0, 1, or 19 ppm fluoride. After a 7‐wk (young) or 13‐wk (adult) cariogenic challenge, the areas of dentinal caries and dentin apposition were quantified after tetracycline staining. Fluoride in the drinking water reduced the progression of dentinal caries and the speed of dentin formation in the young animals but only the progression of dental caries in the adult ones. Both the progression of carious lesions in the dentin and the rate of dentin apposition were 10 times faster during primary dentinogene