Ammonia (NH3) or its ion form ammonium (NH4+) is an important source of nitrogen for living systems. It is synthesized through amino acid metabolism and is toxic when present at high concentrations. In the liver, ammonia is converted to urea through the urea cycle. Elevated levels of ammonia in the blood (hyperammonemia) have been found in liver dysfunction (cirrhosis), while hypoammonemia has been associated with defects in the urea cycle enzymes (e.g. ornithine transcarbamylase). Simple, direct and automation-ready procedures for measuring NH3 are popular in research and drug discovery. This ammonia assay is designed to directly measure NH3 and NH4+. In this assay, NADH is converted to NAD+ in the presence of NH3, ketoglutarate and glutamate dehydrogenase. The decrease in optical density at 340 nm or fluorescence intensity at lambda ex/em = 360/450 nm is directly proportionate to the NH3 concentration in the sample.