The recent demonstration of the endothelial fenestration at the juxtaglomerular part of the afferent arteriole facing mesangial and granular cells and finger-like protrusions of urinary space into the region of the lacis in experimental animals like the rat, the mouse, and Tupaia belangeri led us to propose the hypothesis of a "short loop" feedback mechanism. To establish whether a further species possesses these morphological features, the JGA region was examined in the human kidney. Tissue was obtained by needle biopsy from patients and conventional electron-microscopical procedures including serial sections were utilized to reevaluate the morphology of JGA. Both the endothelial fenestration in the wall of the afferent arteriole and protrusions with filtration slits of urinary space into the lacis, were observed with remarkable heterogeneity. The occurrence of these features of JGA in such diverse mammalian species as rat, mouse, Tupaia, and man suggests that these structures may be involved in regulatory process and the proposed "short loop" feedback mechanism may be a general phenomenon.