The vascular system of a rat mammary tumor, fibroadenoma, was investigated by scanning electron microscopy of resin casts. The present research was designed to provide a three-dimensional presentation of the tumor vasculature and its capillary circulation. Arterial injections of methyl methacrylate were made through the left ventricle to prepare the corrosion casts. The principal sources of blood to the tumor at the breast and inguinal region were the lateral thoracic and caudal superficial epigastric arteries, respectively. These vessels entwined the tumor, establishing irregular vascular conglomerations over its entire surface. The main vessels to the tumor were always straight and greatly enlarged just before and after they entered the tumor. The main trunks of the tumor vascular vessels lacked intraarterial cushions, which regulate blood flow at branching sites. The capillary network consisted of a plexus of fine capillaries with no resemblance to the differentiated vascular system. Two types of capillaries were present in the network: those that were thin with a luminal diameter of 5-12μm and formed a coarse capillary plexus, possibly the fibrous part, and those that were thick with sinusoidal dilations ranging 40-50μm in diameter and formed a densely packed capillary network, possibly the glandular part.