Recently it was shown that the formation of nitric oxide (NO) is increased in biological tissues during ischemia, and that there is an enzyme-independent pathway of NO generation due to reduction of tissue nitrite under the acidic conditions which occur. To investigate the quantitative importance of this mechanism of NO generation in biological systems where pH and nitrite concentrations vary, electron paramagnetic resonance and chemiluminescence studies were performed to characterize the kinetics and magnitude of the nitrite disproportionation process. The reaction process and the corresponding rate law of NO formation from nitrite were derived. The generation and accumulation of NO from typical nitrite concentrations found in biological tissues increased 100-fold when pH decreased from normal values of 7.4 to the acidic values found in ischemic tissues, such as the heart, where pH falls to 5.5. It was also observed that ischemic heart tissue contains reducing equivalents which reduce nitrite to NO, further increasing the rate of NO formation more than 40-fold. Under these conditions the magnitude of this enzyme-independent NO generation may exceed that which can be generated by tissue concentrations of nitric oxide synthase. Thus, in ischemic tissues nitrite can be primarily a source rather than a product of NO.