Direct examination of the structure and function of the macula densa is compromised by the relative inaccessibility and small size of this cell plaque. We report the isolation and perfusion of rabbit nephron segments with attached glomeruli and the direct visualization of the macula densa with differential interference-contrast microscopy. We used this technique to examine the structural sensitivity of the macula densa to perturbations in luminal osmolality and NaCl concentration. Reducing luminal osmolality from 290 to 70 mosmol/kg by removing NaCl resulted in a dilation of the lateral intercellular spaces that was both reversible and specific to the region of the macula densa. Associated with the dilation of the intercellular spaces was a small (congruent to 10%), but statistically significant, increase in the height of the macula densa cells. These structural events were related to the reduction in luminal osmolality, since isosmotic replacement of NaCl with mannitol resulted in no detectable structural changes. Thus, the macula densa may represent a small water-permeable plaque of cells within the remaining water-impermeable thick ascending limb of Henle's loop.