After a group of autoptic kidneys, taken from new-born babies, had been injected via the ureter with micronised barium at various pressures, they were examined micro-radiographically and histologically in order to observe the appearance of intra-renal reflux and the anatomical structures involved. The radiological patterns, clinically observed during cysto-uretrography, were also considered. The intra-renal reflux is tubular, when injection is carried out at a low pressure, whereas at higher pressures one can observe the erosion of the fornices of the calyces and the following sino-lymphatic and/or venous drainage.