The neuronal coding of stimulus-to-action sequences is believed to involve the release of dopamine (DA) and norepinephrine (NE). The electrochemical similarity of these monoamines, however, confounds real-time measurements of their release. Here we report cell-based neurotransmitter fluorescent engineered reporters (CNiFERs) that use the specificity of G protein-coupled receptors (GPCRs) to discriminate nanomolar concentrations of DA and NE. CNiFERs were implanted into the frontal cortex of mice to measure the timing of neurotransmitter release during classical conditioning with the use of two-photon microscopy. The onset of DA release correlated with that of licking and shifted from the time of the reward toward that of the cue upon conditioning. In contrast, concurrent release of NE did not correlate with licking or the cue. This generation of CNiFERs provides unique tools to assess the release of monoamines. The molecular design of these CNiFERs may be generalized to realize CNiFERs for any molecule that activates a GPCR.