ACCOUNTS have been already published of the effects of vitamin D, alteration of the Ca : P ratio of the diet, and dietary restriction, upon the dentin of the incisors of rachitic rats1. It was noted during this work that the formation of organic enamel was sometimes upset by these procedures in animals on the Steenbock and Black rachitogenic diet of high Ca : P ratio; but that the organic enamel of animals made rachitic by low Ca : P ratio diets was scarcely ever affected. The fact that organic enamel formation in rats on a high Ca : P ratio diet is easily upset by various metabolic changes has been confirmed by recent experiments, which were undertaken in a different way and originally for another purpose2 : after twenty-eight days on the usual Steenbock and Black diet, the animals were subjected to dietary restriction for 5–6 days. By this time it was found by examining a control rat that the epiphyses showed a well-marked 'line test' response. The remainder of the litter was then placed back on to full Steenbock and Black diet, and rats were killed and examined at intervals up to fifteen days thereafter, depending on the litter size and length of survival. Five litters of rats were treated in this way.