Eighten black and white weanling rats were divided equally into three groups, and each group received a diet which was either adequate in all respects, deficient in calcium only or in calcium and vitamin D. After 7 weeks each rat received an intra‐peritoneal injection of 3H‐proline (5μ Ci/g body weight; 578 m Ci/mM) and was sacrificed 5 hours later. The buccal mucosa and the attached gingiva were removed, the unbound labelled proline removed by dialysis and salt soluble and insoluble tissue fractions were separated by dialysis and filtration. Each fraction was gelatinised then hydrolysed and hydroxyproline estimation and tritium counting were carried out on the hydrolysate.Larger amounts of insoluble than of salt soluble collagen were recovered. Labelled proline was incorporated in measurable amounts into the insoluble collagen in the relatively short interval before sacrifice. The specific activities of the salt soluble collagen were greater than those of the insoluble collagen and showed greater variation within each dietary group.The rate of incorporation of labelled proline into insoluble oral collagen in rats deficient in calcium was significantly (p = 0.01) greater than normal and was significantly decreased (p = 0.01) in rats deficient in calcium and vitam