a monitor for analyzing a body fluid for diffusable chemical substances has a liquid filled probe , for example a catheter , having a region contactable with the body fluid within the body and permeable to chemical substances of interest . a sensor unit is in liquid communication with the probe and has sensors , each with an output dependent on the presence of a chemical substance of interest . a periodic analysis of the probe liquid is provided from the sensor unit and a pumping system is adapted to circulate the probe liquid around a closed liquid flow path , including the permeable region of the probe , a number of times in each period between the analysis of the liquid .