Cells use gaseous molecules such as nitric oxide (NO) to transmit both intracellular and intercellular signals.In principle,the endogenous small molecules regulate physiological changes,b ut it is unclear howr andomly diffusive molecules trigger and discriminate signaling programs. Herein, it is shown that gasotransmitters use time-dependent dynamics to discriminate the endogenous and exogenous inputs.F or ar eal-time stimulation of cell signaling, we synthesized ap hoto-cleavable metal-nitrosyl complex, [Co III -(MDAP)(NO)(CH 3 CN)] 2+ (MDAP = N,N'-dimethyl-2,11diaza[3,3](2,6)pyridinophane), which can stably deliver and selectively release NO with fine temporal resolution in the cytosol, and used this to study the extracellular signal-regulated kinases (ERKs), revealing howc ells use both exogenous and endogenous NO to disentangle cellular responses.T his technique can be to understand how diverse cellular signaling networks are dynamically interconnected and also to control drug delivery systems.