Placental growth dynamics are an important, but poorly understood aspect of placentation for which suitable animal models are lacking. Although the guinea pig is recognised as a valuable model for human pregnancy, its suitability for this purpose is not apparent, since the developmental and evolutionary processes that led to its lobulated placenta are not fully understood. Therefore, a study on immunohistochemistry has been undertaken, including the antibody mib-1 against protein Ki-67 as a proliferation marker. It was found that cellular trophoblast derived from the placental margin is the major centre of proliferation, supported by a moderate number of proliferating cells inside the labyrinth. In contrast to the ancestral condition of caviomorph rodents, the evolution of significant placental lobulation requires the latter source of proliferation. The distribution pattern of proliferating trophoblast cells revealed for the guinea pig strongly resembles the distribution in the human placenta in that (a) evenly distributed, moderate numbers of cells occur within the exchange area and (b) focal accumulations of proliferating trophoblast cells are present along the fetal mesenchymal border and appear to serve as a growth zone for appositional growth of new exchange tissues. Thus, the guinea pig appears to be an appropriate animal model for placental growth processes in human pregnancies.