Diurnal measurements of hydroxyl and hydroperoxy radicals (OH and HO2) made during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer intensive of 1998 indicate that these key components of gas phase atmospheric oxidation are sustained in significant amounts throughout the night in this northern forested region. Typical overnight levels of OH observed were 0.04 parts per trillion (pptv) (1.1¿106 molecules/cm3), while HO2 concentrations ranged from 1 to 4 pptv. Results of diagnostic testing performed before, after, and during the deployment suggest little possibility of interferences in the measurements. Collocated measurements of the reactive biogenic hydrocarbon isoprene corroborate the observed levels of OH by exhibiting significant decays overnight above the forest canopy. The observed isoprene lifetimes ranged from 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidation by the measured OH abundances. Possible dark reactions that could generate such elevated levels of OH include the ozonolysis of extremely reactive biogenic terpenoids. However, in steady state models, which include this hypothetical production mechanism, HO2 radicals are generated in greater quantities than were measured. Nonetheless, if the measurements are representative of the nocturnal boundary layer in midlatitude temperate forests, this observed nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low-NOr environments. ¿ 2001 American Geophysical Union