Cells of the macula densa from sodium-depleted, sodium-loaded and normal rabbit kidneys were investigated morphometrically by light and electron microscopy. The total macula densa volume constituted 0.14 per cent of the cortical volume and was unchanged during altered sodium balance. Ultrastructurally the macula densa cells appeared similar at the three different levels of sodium intake investigated. However, the relative volume of Golgi apparatus in the cells of the macula densa decreased from the normal value of 1.54 per cent of the cytoplasmic volume to 1.25 per cent during sodium-load, indicating that functional changes take place in the cells of the macula densa under these conditions. The volume of the Golgi apparatus was unchanged in the sodium-depleted group. The volume of the cytoplasm, the nuclei and the intercellular space which normally constitute 56 per cent, 28 per cent and 15 per cent of the total volume of the macula densa, respectively, were unchanged during sodium-load and sodium-depletion. The mitochondria which normally constitute 25 per cent of the cytoplasmic volume were similarly unchanged.