Two bed media were tested (gravel and Filtralite) in shallow horizontal subsurface flow (HSSF) constructed wetlands in order to evaluate the removal of ammonia and nitrate for different types of wastewater (acetate-based and domestic wastewater) and different COD/N ratios. The use of Filtralite allowed both higher mass removal rates (1.1 g NH(4)-N m(-2) d(-1) and 3 g NO(3)-N m(-2) d(-1)) and removal efficiencies (>62% for ammonia, 90-100% for nitrate), in less than 2 weeks, when compared to the ones observed with gravel. The COD/N ratio seems to have no significant influence on nitrate removal and the removal of both ammonia and nitrate seems to have involved not only the conventional pathways of nitrification-denitrification. The nitrogen loading rate of both ammonia (0.8-2.4 g NH(4)-N m(-2) d(-1)) and nitrate (0.6-3.2 g NO(3)-N m(-2) d(-1)) seem to have influenced the respective removal rates.