Objective The regulatory mechanism of formation of the countercurrent multiplier in the outer medulla of the kidney is very complex,which has close relation with the epithelial transport and the spatial arrangement of the Henle,s loops and vessels.The objective of the present study was to investigate the tubular course of the short loop nephrons (SLN)in outer medulla. Methods Three C57/BL/6J mice were fixed by perfusion and embedded in Epon 812.Tissue blocks were cut perpendicular to the longitudinal axis of the kidney. A total of 1 200,2.5-μm-thick serial sections were obtained from the surface to the transition of the inner medulla and the outer medulla.After image recordings and alignment, 120 SLN originated from the superficial cortex and mid-cortex were traced and 3D-reconstructed with a series of computer programs written in C. Results SLN (53%) derived from the superficial cortex had their loop bends located at nearly the same level in the middle part of the inner strip of the outer medulla(ISOM). The bend was covered entirely by descending thin limb epithelium whereas SLN(47%) derived from the middle cortex had their loop bends located in various levels in the inner half part of the inner strip of the outer medulla,and the loop bend was covered with the thick ascending limb epithelium.The thick ascending limb epithelium also covered various distances in the descending limb to form a pre-bend segment with about 50-450 μm long.The deepest bend reached the boundary of the outer and inter medulla,where the loop ran winding. Conclusion The difference of both distribution and the epithelium of loop bends in ISOM suggests the differential reabsorption in ultra filtration composition in different levels in ISOM,which may contribute the formation the increase osmotic gradient in ISOM.In addition,the finding that both distribution and the epithelium of the loop bend are associated with corresponding glomeruli suggests that there may be a systemic regulation between the filtration of glomerul