Peroxynitrite, a transient reactive oxygen species (ROS), is believed to play a deleterious role in physiological processes. Herein, we report a two-photon ratiometric fluorescent probe that selectively reacts with peroxynitrite yielding a >200-fold change upon reaction. The probe effectively visualized fluctuations in peroxynitrite generation by arginase 1 in vivo and in vitro. This provides evidence that arginase 1 is a critical regulator of peroxynitrite.