A highly sensitive OH measurement instrument has been developed. It is based on laser-induced fluorescence (LIF) detection of OH using the A2&Sgr;+ v'=0-X2&Pgr; v″=0 transition at 308.15 nm at low pressure. The LIF instrument detects OH directly and with high specificity, a fact that was demonstrated by recording laser excitation spectra (Q1(3), Q21(3) and P1(1) lines) of ambient OH. For high time resolution (typ. 60--100 s), the laser wavelength was modulated on-/off-resonance with the P1(1) line. Here, we report some of the OH measurements obtained by this technique during its first application in a tropospheric field campaign (''POPCORN''), which was conducted in August 1994 in a rural environment in the North-East of Germany. These include diurnal OH concentration profiles with maximum OH concentrations up to 1.4¿107 cm-3 at noon. Minimum OH concentrations were measured in the morning and evening down to the detection limit of (3--6)¿105 cm-3 (SNR=2, measurement time 1 min.). During the day, OH fluctuations were observed on a time scale of minutes and hours. These were highly correlated to the flux of the solar UV radiation which is responsible for the primary OH production by photolysis. ¿ American Geophysical Union 1996