Methods and devices for the diagnosis and treatment of diabetes are disclosed in which an analyte concentration within a peritoneal fluid of a human subject may be determined by implanting an analyte sensor apparatus in the subject where the apparatus may comprise a housing and a flexible sensing catheter which has a lumen with a plurality of apertures and an exterior surface with an analyte sensor affixed thereto. The catheter may comprise a proximal end attached to the housing and the remaining end may be positioned freely within the peritoneal space to contact peritoneal fluid where an analyte concentration in the peritoneal fluid may be sensed. The housing may be anchored at a subcutaneous site proximate the peritoneal space. The sensed analyte concentration may then be transduced into a transmittable electrical signal.