Providing seamless and acceptable Quality of Service (QoS) support for multimedia services in heterogeneous environments implies an adaptive QoS system, capable not only of establishing end-to-end QoS support at session setup, but also of reacting to changes dynamically occurring during the session. In this paper, we propose a model for the dynamic negotiation and adaptation of QoS parameters for Networked Virtual Reality (NVR) services, considering such services to be among the most complex type of multimedia. In addition, we propose generic client and service profiles used in the negotiation process. While the model is independent of the particular service and network scenario, certain elements are influenced by QoS requirements specified for the Universal Mobile Telecommunications System (UMTS). The model is implemented in a laboratory testbed and a case study is described to verify functionality.