An implantable medical device and corresponding system and method are disclosed having programmable operating functions for monitoring a patient condition or delivering a therapy. The device has a primary operating function and a trial operating function. Those functions are enabled or disabled in a time dependent manner to permit their comparison and evaluation in a patient. The device may include a telemetry transceiver for receiving external programmable sequences of operating instructions and for transmitting data externally to a programmer. The device may further include a memory for storing the sequence of operating instructions and for storing data related to the delivery of the therapy or the monitoring function.