The Connection Oriented Data Service (CO service) has been defined by the IEEE 802.6 committee to support variable-bit-rate (VBR) data services over DQDB MANs. The main aim of the CO service is to be able to guarantee the QoS required by a wide variety of VBR services. In this paper, we derive an adaptive bandwidth allocation control mechanism for the DQDB CO Data service. Simulation results demonstrate the effectiveness of the proposed mechanism in providing the guarantees on the throughput required by the applications under changing network conditions.