Previous studies have indicated that the teeth of experimental animals show cer­ tain variations which may be related to the amount of chewing required by the foods eaten. Thus the molars of rats which were maintained on a liquid diet tended to show a decrease in specific gravity1 and the molars of rhesus mon­ keys fed a soft diet showed a significant increase in water content.2 Other experiments3 indicated that in rhesus monkeys which were made to chew unilaterally, intraperitoneally injected radioactive phosphorus was taken up to a significantly greater degree in the molars of the nonchewing side than in the molars of the chewing side of the same animal. In the experiments mentioned, activity was measured from the enamel surface in situ with a small end-window counter. The limitations of this method have been pointed out.3 In the work described here the teeth were extracted and the radioactivity was measured first from the enamel surface. Each tooth was then digested and the radioactivity of the whole tooth deter­ mined in solution. These procedures were designed to make the measurements in­ dependent of background influences of the other oral tissues.