We reinvestigated the still controversial existence of arterial pathways by-passing glomeruli within kidneys of rats from weaning to more than 12 months old (i.e., body weight ranging from 39 g to 643 g). For this purpose, the arterial injection of microspheres 7.5 micrometer to 17 micrometer in diameter was combined to corrosion-replication of the arterial bed of a vasodilated perfused kidney preparation. This procedure allowed easy detection of arterial by-passes with light microscope and detailed observation with scanning electron microscope. Our results clearly demonstrate the existence of various categories of arterial by-passes throughout renal cortex regardless of age. Some of them had never been described before. These vascular by-passes were found with increased frequency from superficial to juxtamedullary cortex. In the latter area, frequency was not age-dependent, and approximately 10% (range 4-22%) of juxtamedullary glomeruli were involved. Data derived from previous microsphere studies would suggest that these structures are (partially) nonfunctioning in basal physiological conditions, but more information is needed to assess their possible functional role in the rat.