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The odd-hydrogen radicals OH and HO2 are central to most of the gas-phase chemical transformations that occur in the atmosphere. Of particular interest is the role that these species play in controlling the concentration of stratospheric ozone. This paper describes an instrument that measures both of these species at volume mixing ratios below one part in 1014 in the upper troposphere and lower stratosphere. The hydroxyl radical (OH) is measured by laser induced fluorescence at 309 nm. Tunable UV light is used to pump OH to the first electronic state (A 2Σ+(v’=1) ← X2Π3/2 (v‘=0)) near 282 nm. The laser light is produced by a high-repetition rate pulsed dye-laser powered with all solid-state pump lasers. HO2 is measured as OH after gas-phase titration with nitric oxide. Measurements aboard a NASA ER-2 aircraft demonstrate the capability of this instrument to perform reliably with very high signal-to-noise ratios (>=30) achieved in short integration times (< 20 sec). [ABSTRACT FROM AUTHOR]