The arteriole-glomerular units in different zones of the human kidney have been studied by combined stereomicro-angiographic and histologic techniques. In the cortical zone the afferent and efferent artérioles of a unit communicate solely by way of the glomerular capillaries. In the juxtamedullary zone the afferent and efferent artérioles of most of the units form continuous medullary vessels, of which the glomerular capillaries are side branches. In the fetal and neonatal kidney such continuous artérioles were found only in the pelvic area. The findings indicate the existence of an extraglomerular shunt of varying capacity for the medullary blood flow.