Ferritin was injected through micropipettes into single proximal tubules of the rat kidney. After different time-intervals the injected tubules were microperfusion-fixed with glutaraldehyde and analyzed by electron microscopy. Five minutes after start of ferritin perfusion most of the absorbed protein was located in small and large apical vacuoles. No ferritin was observed in the cytoplasm outside the vacuoles. In the tubule lumen ferritin molecules were associated with the outer electron dense coating of the apical cell membrane invaginations. Sixteen to 20 minutes after start of perfusion most small apical vacuoles were devoid of ferritin. The observations indicated that ferritin was transported to the large apical vacuoles via small apical vacuoles, which represented pinched off plasma membrane invaginations. There was no indication that ferritin crossed the wall of the proximal tubule by passing between the cells or that the ferritin-containing vacuoles emptied into the peritubular space.