In the present study the effect of L-azetidine-2-carboxylic acid on uptake of (3)H-proline into incisor dentin of young rats is investigated. For the experiment, (3)H-proline (50mCi/g) and L-azetidine-2-carboxylic acid (1mg/g) were simultaneously injected into rats. The first rat was killed 10min, the second one 30min, the third one 1hr, the fourth one 6hr, and the last one 24hr after the injection. The upper and lower incisors were used for light and electron microscopic autoradiography. In the control, the same procedure was performed but with an injection of 50mCi/g of (3)H-proline only. Quantitative analysis of (3)H-proline incorporation into dentin and pulp of the upper incisors was done and radioactivity was counted by a liquid scintillation-counter. In the control group, at 10 and 30min after the injection, grains were found in odontoblasts. At 1 and 6hr after the injection, they were distributed in the predentin, and the grains were in the dentin 24hr after the injection. In the treated group, on the other hand, grains were found in odontoblasts at 30min and 6hr. Moreover, the grains in the treated group were distributed rather sparsely. The result of the quantitative analysis by a liquid scintillation-counter indicated that the amount of (3)H-proline incorporated into dentin in the treated group was about one third of that in the control. No significant difference was found in the pulps between the treated and the control animals.