Dissolved inorganic N fluxes, including nitrification and denitrification rates, were measured in situ under dark and light conditions using epilithic biofilms from a 6^th to 8^th Strahler-order reach of the River Garonne, France. A broad range of epilithic biofilm biomasses (6–51 g ash-free dry mass [AFDM]/m²) was assessed in 39 benthic chamber experiments. Nitrification was detected in 31 cases (0.1–7.7 mg N m⁻² h⁻¹) and caused a net release of NO₃^– in spite of algal assimilation in light conditions in 5 cases. Denitrification (D_w) (0–8.1 mg N m⁻² h⁻¹), which accounted for 83% of gross NO₃^– removal in the dark, was a significant, permanent N sink. Calculation of mean (±1 SE) N budgets indicated epilithon release of inorganic N in the dark (1.5 ± 0.3 mg N m⁻² h⁻¹) that was significantly related to biofilm mass. Calculation of mean N budgets under light indicated net uptake (1.8 ± 0.4 mg N m⁻² h⁻¹). However, a net release was observed when biomass was >40 g AFDM/m². A calculated daily budget showed evidence for a threshold between photoautotrophy (N assimilation) and heterotrophy (N mineralization) of 23 g AFDM/m². This threshold approximated mean biomass at low water and may correspond to an equilibrium value for epilithic biomass in the river. A conceptual scheme of the epilithic biofilm function and the consequences on the water-column N content in rivers is proposed.