Previous studies. using electron probe microanalysis (EPMA) have shown that several elements are selectively concentrated in the lysosomes of the proximal tubule cells (PTC) of the kidney. In these lysosomes the elements are precipitated as nonsoluble phosphate salts, before being eliminated with the urinary flow as submicroscopic particles. However, in these studies, large. sub-toxic doses were administered, and this particular mechanism of "concentration-precipitation-elimination" was not initially considered as a physiological process, but, only as a pathological consequence of the toxic doses. Due to the high sensitivity of secondary ion mass spectrometry (SIMS), images can be obtained representing the distributions of these elements at a concentration several orders of magnitude lower than that required by EPMA. Two elements. aluminum and indium, have been administered in the rats at very low doses. Analytical images were acquired using the University of Chicago scanning ion microprobe, and it has been shown that in a few hours, the lysosomes of the PTC are able to remove these two elements from the extracellular fluid, even when they are at a concentration at the ppm range in the plasma. This mechanism of "concentration-insolubilisation-elimination" can be considered as a physiological process for the two studied elements. (C) 2004 Elsevier B.V. All rights reserved.