The paper considers the early period of the development of cult architecture in South Mesopotamia. The main source explored is the excavation material from Abu Shahrein and Warka compared with contemporary monuments of Central and North Mesopotamia, as well as with monuments of earlier and later periods of Mesopotamian history. Nineteen layers of Eridu demonstrate step-by-step development of the «nuclear» area first of Ubaid, and later of Sumerian culture from the VI mill. BC on. Both in Warka and in Abu Shahrein traditions of religious architecture are to be seen not only in the continuity of basic characteristics from the earliest cult buildings up to the later temples, but also in the choice of the building site. The location in a special part of the settlement, the platform, tripartite layout, symbolic pilasters and niches decorating the walls, alters and podium on the central axe of the rooms, special equipment for burning the sacrifices and strict symmetry of the temples in Uruk are explicitly analogous to the contemporary cult buildings of later Ubaid period in Eridu. In the course of the subsequent spread of Ubaid culture these characteristic elements were accepted in central and northern parts of Mesopotamia. These very elements, remarkably, constituted the basis of architectural tradition of temple-building in later periods of ancient Mesopotamia.