Levels of OH and peroxy radicals in the atmospheric boundary layer at Summit, Greenland, a locationsurrounded by snow from which HOx radical precursors are known to be emitted, were deduced usingsteady-state analyses applied to (OH + HO2 + CH3O2), (OH + HO2), and OHHO2 cycling. The resultsindicate that HOx levels at Summit are significantly increased over those that would result from O3photolysis alone, as a result of elevated concentrations of HONO, HCHO, H2O2, and other compounds.Estimated midday levels of (HO2 + CH3O2) reached 3040 pptv during two summer seasons. Calculated OHconcentrations averaged between 05:00 and 20:00 (or 21:00) exceeded 4 * 10^6 molecules cm^3,comparable to (or higher than) levels expected in the tropical marine boundary layer. These findingsimply rapid photochemical cycling within the boundary layer at Summit, as well as in the upper pore spacesof the surface snowpack. The photolysis rate constants and OH levels calculated here imply that gas-phasephotochemistry plays a significant role in the budgets of NOx, HCHO, H2O2, HONO, and O3, compounds thatare also directly affected by processes within the snowpack.