This paper presents an architecture and implementation for a dynamic OGSA-based grid service architecture that extends GT3 to support dynamic service hosting - where to host and re-host a service within the grid in response to service demand and resource fluctuation. Our model goes beyond current OGSI implementations in which the service is presumed to be pre-installed at all sites (and only service instantiation is dynamic). In dynamic virtual organizations (VOs), we believe dynamic service hosting provides an important flexibility. Our model also defines several new adaptive grid service classes that support adaptation at multiple levels. Dynamic service deployment allows new services to be added or replaced without taking down a site for reconfiguration and allows a VO to respond effectively to dynamic resource availability and demand. The preliminary results suggest that the cost of dynamic installation, deployment, and invocation, is tolerable.