An implantable access device for allowing repeat access to a site, space, device, or other object, fluid, tissue or region within the body of a patient. The implantable device, in one embodiment, includes a housing having an elongated open guidance channel that leads to an entrance orifice, a valve assembly communicating with the entrance orifice, and an exit orifice. The device permits access via the percutaneous insertion of an accessing filament such as a needle through the entrance orifice and into the valve assembly, which opens to allow the passage of fluids or other filaments such as guide wires or optical fibers. The device can be used for the introduction of therapeutic agents, for the infusion or withdrawal of fluids, or for the introduction of sensing, sampling, or treatment devices to another implanted device or to regions within the patient.