To investigate the permeability properties of the basement membrane beneath macula densa cells and between extraglomerular mesangial cells, thick ascending limbs with attached glomeruli were dissected from rabbit kidney and incubated in a Ringers solution containing either horseradish peroxidase (HRP) (1 mg/ml; molecular weight ~40,000) or native or cationic ferritin (both at 5 mg/ml molecular weight ~450,000) for 5 and 20 min at room temperature. Tubules were processed for electron microscopy. At both time points, HRP reaction product fully permeated the matrix material between extraglomerular mesangial cells, the basement membrane underneath the macula densa, and also was occasionally located in intercellular spaces and intracellular vesicles within macula densa cells. Similar results were obtained with native and cationic ferritin. In separate experiments, thick ascending limbs with attached glomeruli were perfused for 20 min at room temperature with a Ringer solution containing HRP (1 mg/ml). HRP was found in tubulovesicular bodies within the apical cytoplasm of macula densa cells again indicating that these cells exhibit endcytotic activity. However, HRP did not gain access to intercellular spaces indicating that the apical junctional complex was impermeable to HRP. These results demonstrate that the macula densa basement membrane and matrix material between extraglomerular mesangial cells is permeable to high molecular weight molecules and suggest unhindered diffusion of water and solutes within this area.