The purposes of this study were (1) to determine whether or not transcytosis of protein takes place in vivo in rabbit proximal tubules, using ferritin as a marker, (2) to follow the time course of intracellular transport of ferritin, (3) to study the organelles involved in the sequential intracellular transport of the protein, (4) to compare the intracellular transport in vivo and in vitro, and (5) to compare the intracellular transport in rabbit proximal tubules with transport in rat proximal tubules. Female albino rabbits were prepared for micropuncture, and individual proximal tubules were microinfused with a bolus of cationic ferritin for 2-3 min. The tubules were fixed by perfusion with glutaraldehyde at different time intervals after exposure to ferritin. Ferritin was located in the endocytic vesicles, vacuoles, dense apical tubules and lysosomes including multivesicular bodies, initially in the apical part of the cells, then progressively with time transported to the basal part of the cells. An organelle not previously described represented by small tubulovesicular structures (< 0.05 microns in diameter) also contained ferritin. This organelle was observed either close to large ferritin-loaded vacuoles and sometimes connected to the latter, or scattered throughout the cytoplasm, initially in the apical part of the cell but later found increasingly in the basal part. Small clusters of ferritin particles were found in the basolateral intercellular spaces. After 15 min, only small amounts were observed but the number of clusters increased with time after ferritin infusion. No ferritin particles were observed in the Golgi region.(ABSTRACT TRUNCATED AT 250 WORDS)