The effects of azide, chloride, and nitric oxide on the copper sites in Pseudomonas stutzeri nitrous oxide reductase have heen examined. Previous results [J.A. Farrar, A.J. Thomson, M.R. Cheesman, D.M. Dooley and W.G. Zumft, FEBS Lett., 294 (1991) 11] indicated that two binuclear centers are present: a CuA-type center and another center designated CuZ. These sites display distinct reactivities towards exogenous ligands. Azide and chloride appear to bind to the CuA centers but do not perturb the spectral features of the CuZ site that are detectable in the semi-reduced state of the high-activity form of nitrous oxide reductase. Azide appears to coordinate to at least one of the copper ions in an altered (CuZ)* site, present in the low-activity form of nitrous oxide reductase. Nitric oxide perturbs the absorption and circular dichroism spectra attributed to the CuZ site, suggesting that NO reacts directly with this site.