Development of the prostate was studied by serial section reconstruction and computer-assisted three-dimensional analysis. A comparison of ductal budding in species of rat and mouse and the human revealed patterns consistent with common developmental characteristics. Ventral, lateral and dorsal lines of epithelial buds, which emanated from the urogenital sinus into the surrounding periurethral mesenchyme, followed ventro-dorsal and cranio-caudal axes. Subsequent branching morphogenesis was associated with specific mesenchymal condensations. These patterns of budding were closely related to the adult lobe architecture in the rodent prostate. In the human fetus, prostate ductal budding exhibited patterns compatible with the current concept of zonal anatomy.