Lanthanum permeability of the tight junction (zonula occludens) in the renal tubule of the rat. The permeability of the tight junctions (zonulae occludentes) in the proximal and distal convoluted tubules and the cortical collecting ducts of the rat were evaluated with a lanthanum tracer. Eight non-expanded and nine volume-expanded rats were studied. Non-expanded animals received 0.02 ml/min of isotonic saline. Volume-expanded animals were infused with isotonic bicarbonate-Ringer's solution at the rate of 0.375 ml/min until 10% of the body weight had been administered. Individual proximal and distal tubules from the two groups of animals were preserved for ultrastructural examination by intraluminal microperfusion with glutaraldehyde-formaldehyde or osmium tetroxide followed by microperfusion with lanthanum hydroxide. Some tubules were initially exposed to 1 mM LaCl₃ before microperfusion fixation. Additional tubules were preserved via drip-fixation in situ and then perfused with lanthanum hydroxide. In volume-expanded animals the pre-expansion values for GFR (1.21 ± 0.08 ml/min), U_NaV (0.133 ± 0.028 µEq/min) and EF_Na (0.08 ± 0.02%) increased to 1.71 ± 0.17 ml/min, 13.211 ± 1.372 µEq/min, and 6.06 ± 1.16%, respectively. The tight junctions of proximal and distal convoluted tubules were permeable to lanthanum while those of the cortical collecting tubules were impermeable. The results were not detectably different in non-expanded or volume-expanded animals; they were independent of the type and osmolality of the fixative. These results provide morphological evidence for the existence of a paracellular shunt pathway for ion and water movement in the proximal and distal convoluted tubules of the rat.