non - invasive methods have been developed for the measurement of no levels in a variety of fluid media , e . g ., body fluids . the present invention embraces the use of a semi - permeable vessel wherein said vessel contains an no reacting substance to trap no diffusing thereinto , and a simple physical or chemical detection method to measure the levels of the end products . since no is a neutral gas molecule , it is capable of diffusing freely across a wide range of biocompatible polymer membranes which exhibit a high permeability to no and other neutral gas molecules , such as o 2 and co 2 , but which are not permeable to charged molecules , such as no 3 - or no 2 - . the latter two compounds are ubiquitously present in body fluids and often interfere with the measurement of no levels . the permeability of selected membranes to no , but not to no 3 - or no 2 - , makes it possible for the bags employed in the practice of the invention to selectively collect no , even in the presence of potentially competing species . the simple , easy and non - invasive methods of the invention will find a variety of uses , e . g ., for diagnosis and monitoring of no overproduction that has been associated with many inflammatory and infectious diseases .