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We estimated subsurface nitrogen (N) retention efficiency of two Mediterranean wetland-streams affected by agricultural runoff, and analysed the influence of hydrology and physicochemical factors such as N load. Subsurface N retention efficiency was high and greater than the surface N retention quantified in a previous study. The studied wetlands clearly functioned as nitrate-N (NO3 (-)-N) sinks (96.5 % and 89.8 %), which was the dominant N form. They were usually total organic-N (TON-N) sinks (49.9 % and 21.6 %), but were ammonium-N (NH4 (+)-N) sinks/sources. Water temperature was an important predictor of NO3 (-)-N retention efficiency, accounting for 59 % and 34 % of explained variance for both wetlands, respectively. This result highlights the importance of biological processes in NO3 (-)-N retention. The studied wetlands did not present "saturation symptoms" at high inflow NO3 (-)-N concentrations (40 mg l(-1)). These small slope wetlands can be compared to riparian wetlands in terms of NO3 (-)-N retention efficiency. The contribution of the subsurface component to the wetland N mass removal rate was low (0.01 % and 0.05 %). Both wetlands removed an average of 1.1 and 6.2 mu g of NO3 (-)-N m(-2) h(-1).