The effect of acute hypertension, induced in rats by intravenous injection of methoxamine chloride (Mexan), on the stria vascularis and spiral ligament was studied electronmicroscopically with the tracer method of horseradish peroxidase (HRP). Considerable extravasation of HRP occurred in the stria vascularis, due to the increased vesicular transport. The leaked HRP spread into intercellular spaces, but was prevented from spreading towards the endolymph by zonulae occludentes between marginal cells and towards the perilymph by zonulae occludentes between basal cells. The reaction product was occasionally found between basal cells. No leakage of HRP from capillaries was observed in the spiral ligament, although some labelled micropinocytotic vesicles were present in the endothelium. It is suggested that, under acute hypertensive conditions, areas of zonulae occludentes bordering the stria vascularis play an important role as a barrier to HRP, whereas capillaries in the spiral ligament themselves act as a barrier to it.