The transendothelial passage of horseradish peroxidase (HRP) and diffusion of HRP into the pericapillary space, injected intravenously into cats, was studied at the light and electron microscopic level in the mucosa of the eustachian tube. The permeability of capillaries in the pharyngeal two-thirds of the tube was higher than in the tympanic one-third though all the subepithelial capillaries were of the continuous type. In the lamina propria of the subepithelium in the pharyngeal two-thirds much HRPwas found 10 min after its intravenous injection, but in the tympanic one-third, it was absent in the perivascular spaces. The capillaries of the pharyngeal two-thirds had many projections and marginal folds inside the capillary lumen, and large pinocytotic vacuoles containing HRP as well as many micropinocytotic vesicles. On the other hand, the capillaries in the tympanic one-third had many micropinocytotic vesicles too, but only a few large pinocytotic vacuoles.