Previously we have demonstrated that cellular autofluorescence when multiphoton excited by 700-800 nm light is primarily due to NAD(P)H. Within the cell, this signal primarily originates from mitochondria where NADH is expected to be concentrated. Studies conducted within the last year have sought to identify the magnitude of the autofluorescence signal which can be attributed to normal mitochondrial function. Two rodent cell lines (EMT6, RBL) and one human cell line (HeLa) have been examined to date. In all three lines cellular autofluorescence is dynamic, changing according to the status of the mitochondria. When the mitochondria are uncoupled or equivalently when substrate is removed, autofluorescence typically drops by approximately 20% from basal levels. In contrast, poisoning the mitochondria with cyanide results in a 2.5 fold increase in signal. Having established that the autofluorescence signal is a dynamic indicator of metabolic state, more recent projects have utilized this endogenous signal as a reporter of mitochondrial health.