Fluorescence microscopy in experimental hexachlorobenzene porphyria in rats has demonstrated porphyrin deposits in the epithelial cells of the proximal renal tubules and the loop of Henle. In the glomeruli, interstitium, distal tubules, and the collecting tubules, no porphyrins were detected. The similarity of this finding with the data on some forms of porphyrinuria and porphyria in man suggests that the proximal tubules and the loop of Henle play an important role in the mechanism of renal porphyrin excretion.