Parallel monitoring of K(+) and protoporphyrin IX (PPIX) is of vital importance because they are not only involved in a variety of biological processes but also closely linked to each other in numerous cellular pathways. However, there are currently no existing methods that can meet the requirements for parallel and in vivo detection of K(+) and PPIX in living organisms. Herein, we demonstrated a functional nucleic acid (FNA)-based technique for parallel monitoring of K(+) and PPIX in living animals. Specifically, the selected G-rich FNA probe was selectively induced to form a parallel G-quadruplex by K(+). The parallel G-quadruplex then remarkably enhanced the fluorescence of PPIX. Thus, by modulating the fluorescence "turn on" with the G-quadruplex and K(+)/PPIX, both K(+) and PPIX could be detected. After validating the developed method for selective and sensitive detection of K(+) and PPIX in vitro, their dynamic changes in living organisms (i.e., living brains and tumors) following various physiological and pathological processes were simultaneously monitored. The current study not only provides a general method for the detection of metal ions and bioactive molecules but also presents a way to investigate their synergistic functions in the regulation of various biological processes. It may also be helpful for improving the imaging and therapeutic efficacy of PPIX and 5-ALA.