SummaryBarium and strontium may be detected histochemically by the formation of reddish compounds following the treatment of tissues with sodium rhodizonate. Calcium does not appear to react under the neutral conditions employed in the test. Treatment with chromate inhibits the reaction with barium, but not with strontium, providing a means for distinguishing between these elements. Permanent histological sections can bc produced in the usual manner, so long as a neutral fixative is employed.Both barium and strontium were detected in the tissues of many insects. They occurred most frequently in the malpighian tubules, less often in the midgut and reproductive organs, and very occasionally in the hindgut and fat body. It is probable that both elements are absorbed in the midgut of most insects. The distribution of barium and strontium was characteristic for each species. When staining occurred it was almost always of granules, which frequently had a characteristically restricted distribution within the cell. Nucleoli stained occasionally, although the remainder of the nucleus seldom reacted.The possible importance of barium and strontium in insect metabolism is discussed. However, the granules detected are regarded as being either reserves for future use or as a form of storage excretion.