An implantable gastrointestinal (GI) stimulation system includes an implantable medical device and at least one stimulus delivery device configured to be placed in one or more lymphatic vessels of a patient, such as the patient's thoracic duct and/or vessels branching from the thoracic duct. In one embodiment, the implantable medical device includes a GI stimulation circuit to deliver electrical stimulation pulses to one or more target regions adjacent to a lymphatic vessel through the stimulus delivery device. In one embodiment, to control obesity, the electrical stimulation pulses are delivered to the organs and/or nerves of the GI tract to create a sensation of satiety and/or to expedite food movement through the GI tract.