The phosphoroclastic system was demonstrated in cell-free extracts of clostridium sporogenes by the production of carbon dioxide, acetyl phosphate, ATP and reduced NAD in the presence of pyruvate. The kinetics of acetyl phosphate production and NAD reduction were investigated. The addition of sodium nitrite to a suspension of C. sporogenes in glucose medium resulted in a rapid decrease in intracellular ATP concentration which was accompanied by an accumulation of pyruvate in the medium. This accumulation of pyruvate was caused by inhibition of phosphoroclastic system by nitrate. Nitrite inhibits this system by reaction of nitric oxide, formed from nitrate, with the non-haem iron of pyruvate:ferredoxin oxidoreductase.