Integration of different components that compose enterprise information systems (IS) represents a big challenge in the IS development. However, this integration is indispensable in order to avoid IS fragmentation and redundancy between different IS applications. In this work we apply service-oriented development principles to information systems. We define the concept of information system service (ISS) and propose a metamodel of ISS (MISS). We claim that it is not sufficient to consider an ISS as a black box and it is essential to include in the ISS specification the information about service structure, processes and rules shared with other services and thus to make the service transparent. Therefore we define the MISS using three informational spaces (static, dynamic and rule).