The biological relevance of tyrosine nitration is a subject of much interest, because extensive evidence supports formation of 3-nitrotyrosine in vivo under a variety of different pathological conditions. Several reagents are likely to be responsible for nitration in vivo, among others peroxynitrite and nitrite in the presence of H₂O₂/peroxidases. In this work we show that also metmyoglobin and methemoglobin can nitrate free tyrosine in the presence of nitrite and H₂O₂. The results of these studies are simulated rather well by using a scheme that comprehends all the possible reactions that can take place in the system. Thus, a good understanding of the factors that determine the yields is achieved. Finally, we demonstrate that the system metMb/H₂O₂/NO₂⁻ can also lead to the nitration of tryptophan and produces, in particular, 6-, 4-, and 5-nitrotryptophan.