The effect of insulin on the collagen metabolism in dental pulp and gingiva was studied using Sprague-Dawley rat and ³H-proline was injected intraperitoneally and then sacrificed according to predetermined schedule. Lower incisor pulp and gingiva were removed immediatedly and hydrolysed with acid. After digestion, proline and hydroxyproline were separated and analysed quantitatively employing thin-layer chromatography and radioactivity of each amino acid was counted by liquid scintillation counter. The label incorporated into lower incisor pulp and gingiva and radioactivity of ³H-hydroxyproline were increased in insulin administered group. And disappearance rate of ³H-hydroxyproline was decreased. Those results indicated that amino acid, I.e. proline transport and collagen biosynthesis were increased in dental pulp and gingiva by insulin administration.