The osmolality and the electrochemical composition of the endolymph, a potassium-rich positively polarized extracellular fluid in the cochlea, was studied in the rat. Endolymph of each cochlear turn was hyperosmotic to perilymph and plasma. Osmolalities (mosmol/kg H2O) were 329 +/- 2.9 (mean +/- SE) (n = 13) in basal turn endolymph, 322 +/- 2.7 (n = 9) in middle turn endolymph, 317 +/- 5.2 (n = 3) in apical turn endolymph, 289 +/- 3.1 (n = 14) in perilymph of the scala vestibuli, and 298 +/- 1.8 (n = 7) in plasma. Moreover, differences in osmolality and electrochemical composition of endolymph, involving resting potential and K and Cl concentrations, were observed between the basal and the middle cochlear turns, suggesting the presence of an electrical and osmotic gradient within endolymph, declining from the base to the apex of the cochlea. The active potassium transport into endolymph, located presumably in the stria vascularis, could account for both the internal and external osmotic gradients.