Projected changes in climate in Southern Norway include increases in summer and autumn precipitation. This may affect leaching of dissolved organic matter (DOM) from soils. Effects of experimentally added extra precipitation (10 mm week⁻¹) during the growing season of 3 years (2004–2006) to small headwater catchments at Storgama (59°0′N, 550–600 m a.s.l.) on leaching of total organic carbon (TOC) and total organic nitrogen (TON) were assessed. Extra precipitation did not have a significant effect on average TOC and TON concentrations in runoff. Thus, fluxes of TOC and TON increased nearly proportionally with water fluxes. This suggests that a store of adsorbed and potentially mobile TOC and TON in catchment soils buffers the concentration of DOM in runoff. The size and dynamics of the pool of TOC and TON depends on the balance between production and leaching rates. Infrequent short droughts had only small effects on TOC and TON fluxes in runoff from the reference catchments.