The respiratory quotient of the stria vascularis was measured in vitro by means of Cartesian diver microgasometry. A value of about 1.2 was found when the incubation medium was phosphate-buffered serum substitute with glucose as the sole substrate. This value suggests that endogenous lipids and amino acids do not contribute significantly to strial respiratory metabolism and that carbohydrate is the primary fuel in vitro. A high activity of the hexose monophosphate pathway may be responsible for raising the respiroatyr quotient above unity.