This present investigation was concerned with the content of collagen and minerals of incisor and molar dentin in rats of differednt ages. The results obtained were summarized as follows. 1. There was no remarked change in the content of collagen-hydroxyproline of incisor and molar dentin according to days after delivery. 2. The Ca content in incisor dentin rose from approximately 15.4% of the dry weight in 7 days after delivery to 21.1% in 22 days while that in molar dentin from 17.6% in 7 days to 19.7% in 22 days. 3. The P content in molar dentin rose from 8.5% in 7 days after delivery to 11.65% in 22 days. while in incisor rose from 8.05% in 7 days to 11.85 in 22 day after delivery.