A slow intracarotid infusion of hypertonic sodium chloride solution, 350 mEq/L, and potassium chloride, 5 mEq/L, was used to increase serum sodium chloride over a 90-minute period in 85 cats. A slow, steady rise in serum sodium occurred, which reached almost 208 mEq/L at the end of 90 minutes. A concomitant rise in sodium occurred in the cerebrospinal fluid and perilymph. In contrast, the sodium showed only a transient slight increase in the cochlear and vestibular endolymph. Endolymph potassium appeared to rise in order to balance out the increase in sodium concentration of the surrounding and extracellular fluids. The ratio of sodium to potassium ions in both the endolymph and perilymph compartment remained relatively constant before and after the infusion with hypertonic NaCl. After infusion, the total concentration of ions in endolymph was similar to that of perilymph. These experiments indicate that the endolymph compartment has a built-in mechanism for maintaining a low-sodium concentration while keeping ionic balance with the surrounding perilymph and serum.