The uptake of horseradish peroxidase (HRP) was studied in rat junctional epithelium (JE). Intravenous HRP (12.5 mg, 25 mg, or 50 mg) was only endocytosed by the entire JE (particularly by the coronal portion of the JE). Although HRP uptake was negligible at a concentration of 12.5 mg, marked uptake was observed at concentrations of 25 mg and 50 mg. However, there was no difference in the HRP uptake by the JE between the concentrations of 25 mg and 50 mg. By electron microscopy, HRP was detected in the organellae (micropinocytotic vesicles, intracytoplasmic vacuoles, dense bodies, and multivesicular bodies) in the junctional epithelial cells (JE cells). HRP-positive micropinocytotic vesicles were detected in all JE cells. However, intracytoplasmic vacuoles, which are characteristically observed in JE cells, were not always positive for HRP. Numerous HRP-negative vacuoles were detected in the JE cells. The number of HRP-positive organellae in the coronal portion of the JE was pronounced, compared with those in other portions. These findings provided evidence that the entire JE, especially its coronal portion, has endocytotic ability by micropinocytotic vesicles, rather than intracytoplasmic vacuoles, for foreign substances.