Hepatic structures appearing during embryonic Carnegie stages 11-23 were analyzed and compared in OFA-IOPS rat and human embryos. The group of rats--crown-rump length (CRL) 2-16 mm, 10-16 days postcoitus--was composed of 127 specimens (52 of stages 11-12, 55 of stages 13-19, and 20 of stages 20-23), the human group of 9 embryos at stages 14-23--CRL 5-31 mm, age 32-57 days--and human stages 11-13 were described according to former literature. The specimens were subjected to serial histological sections with graphic reconstructions. In both series, stage 11 was characterized by hepatic diverticulum development, stage 12 and thereafter by cellular differentiation (septum transversum giving the liver stroma and hepatic diverticulum the hepatic trabeculae), and stage 13 by epithelial cord proliferation enmeshing stromal capillaries. From stage 14, the hepatic gland and its vascular channels presented considerable enlargement while hematopoietic function appeared. From this stage, the development of cystic primordium, never present in rat, was constant in man. At stage 18, after a period of obturation due to epithelial proliferation, the bile ducts became reorganized and ensured the continuity between liver cells and gut. From stages 18 to 23, biliary ductules developed in periportal connective tissue producing ductal plates that received biliary capillaries. Except for gallbladder, similarity and presence of the same hepatic structures in man and rat during the embryonic period stages 11-23 permit us to consider the rat as a good experimental model for liver development, for example, in studies on teratology and congenital anomalies.