A method of placing an electrode in a peritoneal cavity including the following steps: inserting an endoscope through a patient's mouth into the patient's stomach; passing the endoscope through an opening in the stomach into the peritoneal cavity; and using the endoscope to place the electrode in contact with tissue within the peritoneal cavity other than stomach tissue. Another aspect of the invention provides a method of electrically mapping tissue accessed from a peritoneal cavity including the following steps: inserting an endoscope through a patient's mouth into the patient's stomach; passing the endoscope through an opening in the stomach into the peritoneal cavity; passing an electrode through the endoscope; placing the electrode in contact with tissue within the peritoneal cavity other than stomach tissue; and applying a mapping stimulation signal to the electrode from a mapping stimulator.