Stratospheric OH and HO2 radical densities have been measured between 36 and 23 km using a balloon-borne, in situ instrument launched from Palestine, TX on August 25, 1989. OH is detected using the laser-induced fluorescence technique (LIF) employing a Cu-vapor-laser pumped dye laser coupled with an enclosed-flow detection chamber. HO2 is detected nearly simultaneously by adding NO to the sample flow to convert ambient HO2 to OH. Observed OH and HO2 densities ranged from 8.0¿2.8¿106 and 1.4¿0.5¿107 molec cm-3, respectively, at 36 km, to 1.4¿0.5¿106 and 3.0¿1.0¿106 at 23 km, where the uncertainty is ¿1&sgr;. The HO2 density exhibits a maximum in the 34--30 km region of 1.7¿0.6¿107. The data were obtained over a solar zenith angle variation of 51¿ at 36 km to 61¿ at 23 km. O3 and H2O densities also were measured simultaneously with separate instruments. ¿American Geophysical Union 1990