The determination of the concentration of OH in the Earth's troposphere is of fundamental importance to an understanding of the chemistry of the lower atmosphere. Many experiments to measure OH concentration have been performed in recent years; very few of these experiments have produced significant results. In particular, because of the extremely low amount of OH in the clean troposphere (several tenths parts per trillion by volume at summertime local noon), none of the experiments performed have attained the sensitivity limit necessary to test the photochemical theories of OH and its temporal behavior. Described here is an experiment utilizing a laser source and very high resolution ultraviolet absorption spectroscopy to measure the concentration of OH in a clean environment at a measured sensitivity limit of approximately 5¿105 cm-3 (0.01 pptv) with an integration time of several minutes. This limit is substantially below predicted noontime OH concentrations and should be low enough to provide a rigorous test of photochemical theories of hydroxyl formation. This paper describes the instrumentation developed to make the measurements. ¿ American Geophysical Union 1992