Small amounts of soluble oxalate in the food or drink produced a hard deposit, which grossly resembled human dental calculus, on the teeth of rats. The extent of this encrustation, which is presumably calcium oxalate, varied with the concentration of oxalate consumed and with the length of the experiment. Because of this phenomenon, oxalic acid solutions having a pH as low as 2.1 did not, in vivo, etch the enamel of the teeth of rats, in contrast to the action of other common food acids (citric, lactic, phosphoric, sulfuric) at even higher pH levels. When oxalic acid or its sodium salt was present in the rat's food or drink the decalcification of teeth, which accompanies ingestion of phosphoric (0.055%) and citric (0.20%) acid solutions, diminished or disappeared. Essentially complete protection against the etching action of orange juice was afforded by a concentration of 0.1% oxalate in the juice. Natural oxalate-containing foods, such as spinach and rhubarb, when incorporated into the diet in a manner which did not remove the soluble oxalate prior to ingestion, produced the characteristic protective films on the molars of rats within 1 week. Inasmuch as these experiments with rats were devised to give clearcut results within a short time, caution must be exercised in interpreting these data with respect to human practices. It is hoped, however, that further studies using foods with moderate levels of “soluble” oxalate may offer some insight into the dietary control of dental calculus and caries initiation.