Nitric oxide synthases (NOSs) catalyze the formation of nitric oxide from L-arginine. We purified the heme containing, tetrahydrobiopterin-free, oxygenase domain of rat neuronal nitric oxide synthase (nNOSox) overexpressed in Escherichia coli. We found catalase activity in nNOSox. This is significant because H2O2 may also be a product of nitric oxide synthases. We found H2O2 assisted product formation from N-hydroxy-L-arginine and even from L-arginine both in the presence and in absence of tetrahydrobiopterin. We propose how heme moiety of the oxygenase domain alone is sufficient to carry out both steps of the NOS catalysis. © 2000 Federation of European Biochemical Societies.