Nitric oxide can be rapidly extracted from dilute aqueous solutions into the gas phase by diffusion through hydrophobic membranes, permitting direct measurement of nitric oxide and observation of its reactions by a commercial chemiluminescent nitric oxide detector. Nitric oxide is a hydrophobic gas, crossing membranes much as oxygen and carbon dioxide do in blood gas analyzers. A loop of hydrophobic tubing made from microporous polypropylene was passed through a solution containing nitric oxide. One end of the tubing was connected to the inlet of the nitric oxide detector and maintained under vacuum. A continuous stream of carrier gas is passed through the other end of the tubing to sweep the nitric oxide from the inside of the tubing walls into the chemiluminescent detector. The device can measure nitric oxide at final concentrations as low as 10 nM with a time response of under 1 s. When the gas flow through the tubing is transiently diverted for periods up to several minutes, nitric oxide continues to accumulate within the tubing. The accumulated nitric oxide is rapidly washed out upon return of gas flow, resulting in a single peak which increases the sensitivity by one to two orders of magnitude. This method of measuring nitric oxide can be used to follow kinetics of nitric oxide formation or reactions with biological molecules. It is substantially faster and more selective than commercial electrochemical probes for nitric oxide.