Extracellular generation of inorganic pyrophosphate (PPi) in cartilage organ culture is markedly augmented by ATP.ATP, not an ATP metabolite (ADP, AMP, adenosine) is necessary for this augmentation. Excess PPi production is effectively blocked by known inhibitors of nucleoside triphosphate (NTP) pyrophosphohydrolase (EDTA, EGTA, dithiothreitol). Excess 32P-PPi is generated directly from gamma 32P-ATP by cartilage, as substrate and product have similar specific activities. These findings strongly favor ecto-NTP pyrophosphohydrolase as the source of extracellular PPi generation in the presence of NTP. Additionally, active nucleotide and nucleoside catabolism is demonstrated in these cartilage organ cultures.