In recent years, research about distributing information sources over a network has become increasingly important. We concentrate on the issues of information cooperation among heterogeneous virtual enterprise information sources. When a virtual enterprise was built up, enterprise information will be bound together with some constraints enforcing virtual enterprise logic across several enterprises. We explore the needs on the constraints in cooperative virtual enterprise information systems, and present a cooperative model suitable for constructing a virtual enterprise. This is done through the constraint definition and constraint model developed in a virtual enterprise information integration system named ViaScope. In order to manage these constraints effectively, the properties of distributed constraints are studied deeply. Further some novel implementation issues in the ViaScope system are provided including the cooperative architecture, constraint evaluation and an active rule based maintenance strategy.