Reactive metabolites and related electrophilic drugs are among the most challenging small molecules to study. Conventional approaches to deconstruct the mode of action (MOA) of such molecules leverage bulk treatment of experimental specimens with an excess of a specific reactive species. In this approach, the high reactivity of electrophiles renders non-discriminate labeling of the proteome in a time- and context-dependent manner; redox-sensitive proteins and processes can also be indirectly and often irreversibly affected. Against such a backdrop of innumerable potential targets and indirect secondary effects, linking phenotype to specific target engagement remains a complex task. Zebrafish targeting reactive electrophiles and oxidants (Z-REX)-an on-demand reactive-electrophile delivery platform adapted for use in larval zebrafish-is designed to deliver electrophiles to a specific protein of interest (POI) in otherwise unperturbed live fish embryos. Key features of this technique include a low level of invasiveness, along with dosage-, chemotype-, and spatiotemporally-controlled precision electrophile delivery. Thus, in conjunction with a unique suite of controls, this technique sidesteps off-target effects and systemic toxicity, otherwise observed following uncontrolled bulk exposure of animals to reactive electrophiles and pleiotropic electrophilic drugs. Leveraging Z-REX, researchers can establish a foothold in the understanding of how individual stress responses and signaling outputs are altered as a result of specific reactive ligand engagement with a specific POI, under near-physiologic conditions in intact living animals.