Chemical reactions occurring within a living cell are measured in a manner that does not affect the viability of the cell or the reaction under study. In one embodiment, one or more sensors are introduced into the cell and/or covalently associated with the exterior cell membrane. The sensor(s) emit an observable signal indicating a value of a parameter associated with the chemical reaction, e.g., the concentration of a reaction product. Because cell viability is not compromised, the cell may be stimulated (e.g., by subjection to an agonist or antagonist, a pathogen, a pharmaceutical compound, or a potential toxin) so as to affect the reaction under study.