A frequently applied photometrical assay of NO is based on the reaction of NO with oxyhaemoglobin. This study shows that peroxynitrite induces spectral changes of oxyhaemoglobin identical with those elicited by NO. Like a variety of other agents, peroxynitrite did not interfere with NO measurements using a Clark-type electrode, demonstrating that electrochemical detection has an advantage over the oxyhaemoglobin method for specific determination of NO.