The right superior cervical ganglia of 11 cats were removed. After intravenous injection of horseradish peroxidase, both sides of the cochleas were infused with 2% glutaraldehyde and the lateral wall of the cochlea was then microdissected. As control group, both sides of the cochleas of 4 cats were extracted while the superior cervical ganglia remained intact. The samples were incubated after a modified Graham-Karnovsky method and observed by light microscopy. The diameters of the capillaries of the stria vascularis and spiral ligaments were measured. In 8 cats (73%), the diameters of the capillaries of the stria vascularis on the side on which sympathectomy was performed were significantly greater than those on the opposite side. There was no statistical difference between the diameters of the capillaries of the spiral ligament on either the sympathectomized side or the intact side, except in 2 cats. In the control group (in which the superior cervical ganglia were left intact), no statistical difference was found as regards the diameters of the stria vascularis capillaries on either the left or right side. In summary, sympathectomy affected only the capillaries of the stria vascularis and not those of the spiral ligament.