Nitrate reduction plays a key role in the biogeochemical dynamics and microbial ecology of coastal sediments. Potential rates of nitrate reduction were measured on undisturbed sediment slices from two eutrophic coastal environments using flow-through reactors (FTR). Maximum potential nitrate reduction rates ranged over an order of magnitude, with values of up to 933 nmol cm(-3) h(-1), whereas affinity constants for NO(3) (-) fell mostly between 200 and 600 microM. Homogenized sediment slurries systematically yielded higher rates of nitrate reduction than the FTR experiments. Dentrification was the major nitrate removal pathway in the sediments, although excess ammonium production indicated a contribution of dissimilatory nitrate reduction to ammonium under nitrate-limiting conditions.