Experimental evidence points to the existence in many or all plants of a substance which affects carbohydrate metabolism. The leaves of the blueberry plant are said to contain this ingredient in relative abundance, in a form which permits of extraction. The physiological action of an extract prepared from blueberry leaves was first investigated by Mark and Wagner. This investigation was soon followed by observations relative to its effect in depancreatized dogs. The variable results obtained by these workers were explained by the demonstration of two antagonistic principles in their preparations. The one tended to raise the blood sugar, the other to lower it. An attempt was made to separate and purify the latter substance, and the resulting product was given the name myrtillin.