Denitrification in exposed intertidal sediment was measured for the first time at locations in the Tagus Estuary, using a new ammonium spray method. With this method,(NH4+)-N-15 was sprayed on to the sediment surface, and nitrification-denitrification was subsequently estimated from accumulation of N-29(2) and N- 30(2) in gas-tight glass chambers placed on the N-15-labeled sediment. Coupled nitrification-denitrification ranged from 8.5 to 11.5 mu mol N m(-2) h(-1) at night to below 2 mu mol N m(-2) h(-1) during the day. Salt stress, light inhibition and depletion of NH4+ by benthic diatoms could contribute to the low daytime activity. The nighttime rates were confirmed by an independent estimate based on potential nitrification- denitrification and oxygen uptake. Coupled nitrification- denitrification activity at night during exposure was not significantly different from the activity measured when the sediment was inundated, but was 3 times lower than rates measured at a subtidal station in the estuary.