The cytochrome cd(1) nitrite reductases are enzymes that catalyse the reduction of nitrite to nitric oxide (NO) in the bacterial energy conversion denitrification process. These enzymes contain two different redox centres: one covalently bound c-haem, which is reduced by external donors, and one peculiar d(1)-haem, where catalysis occurs. In the present paper, we summarize the current understanding of the reaction of nitrite reduction in the light of the most recent results on the enzyme from Pseudomonas aeruginosa and discuss the differences between enzymes from different organisms. We have evidence that release of NO from the ferrous d(1)-haem occurs rapidly enough to be fully compatible with the turnover, in contrast with previous hypotheses, and that the substrate nitrite is able to displace NO from the d(1)-haem iron. These results shed light on the mechanistic details of the activity of cd(1) nitrite reductases and on the biological role of the d(1)-haem, whose presence in this class of enzymes has to date been unexplained.