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We describe an instrument that is used for spatially homogeneous, submillisecond application of low‐molecular‐weight substrates (hundreds of daltons) in the neighborhood of a single biological cell and the measurement of the subsequent response of the cell to the substrate. Substrate application is accomplished by ultraviolet photolysis of caged substrate. Cell response is measured, in our particular case, as current flux into the cell in response to membrane conductance changes caused by substrate binding to cell surface receptors. We present data that show fast channel‐opening kinetics of nicotinic acetylcholine receptors on the surface of a BC3H1 cell, a transformed cell line from mouse brain, and we discuss the measurements of ultraviolet‐induced cell damage and radiation fluence at the cell surface. The technique described is also useful for nonbiological studies requiring known fluences applied to microscopic samples.