MNI-caged-L-glutamate, a derivative of glutamate conjugated with a 4-methoxy-7-nitroindolinyl (MNI) photoprotective group, remains pharmacologically inactive at neuronal glutamate receptors at concentrations up to mM. Upon light exposure (300 - 380 nm excitation), it rapidly releases L-glutamate by cleaving the MNI group within submicroseconds. This characteristic enables the investigation of fast synaptic glutamate receptor mechanisms in situ.