Systems and methods for treating a gastrointestinal condition of a patient includes minimally invasively implanting a modular stimulator system in a patients treatment site. The modular stimulator system comprises a microstimulator module, a small power source module and a macrostimulator module that are detachably attachable to each other. For the first phase of treatment, the microstimulator module and power source are implanted with a stimulating electrode proximate a target tissue. Electrical stimulation is provided for a short period of time and treatment efficacy is evaluated through clinical results and patient reporting. If therapy is not effective, stimulation parameters can be changed and/or the implantation site can be moved. If therapy is effective, portions of the microstimulator module and/or small power source are replaced with the macrostimulator module for long term therapy of the second phase of treatment.
