Oxidized Pseudomonas nitrite reductase (Ps.NiR) reacted with nitric oxide (NO) forming mostly Comp. I at the heme c and heme d moieties in the acidic pH region (+. However, Comp. II was formed at heme d to some extent between pH 6 and 7. Comp. II (d) was also formed by direct reaction of reduced Ps.NiR with NO below pH 7, whereas Comp. II (c) appeared only below pH 6. These results could be explained as being due to a higher affinity of the reduced heme d than of the reduced heme c toward NO. In the pH region higher than neutral the reduced hemes remained unattacked except at extreme alkaline pH. When sodium nitrite was added anaerobically to the reduced enzyme in the presence of NADH and PMS, Comp. II (c) appeared even at (and below) pH 7.2, whereas above that pH the reduced heme was predominant. Consequently it was concluded that under these turnover conditions Comp. II (c) was formed actually from Comp. I (c) by accepting an electron, but not by the direct reaction of reduced heme c with NO. The same mechanism could also explain the behavior of the heme d moiety, thus being in accord with the reaction scheme for Ps.NiR proposed previously (1).