The role of calcium in the keratinization process has been studied in hypocalcaemic rats after surgical removal of the parathyroids. Intraperitoneal injection of Na2(35) SO4 allowed the study of the incorporation of sulphur in palatal and gingival epithelium. After parathyroidectomy, the calcaemia decreased strongly in the first week, reached its lowest value at the end of the second week and then increased progressively. In the control rats, the concentration of the silver grains increased from the basal layer to the stratum corneum, whereas in the parathyroidectomized rats the grains were concentrated in the basal layers. The variations of the values of the calcaemia and the number of grains were in correlation for the palatal and gingival epithelium. This correlation seemed to be confirmed for the palatal epithelium by the results obtained by radioactivity measurements. A relation between the calcium plasmatic concentration and the sulphur incorporation seemed to be confirmed. The hypocalcaemia was accompanied by diffusion anomalies of sulphur and glycoaminoglycans sulphation.