The seasonal variation and depth distribution of the capacity for denitrification and dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (NO ₃ ⁻ ammonification) were studied in the upper 4 cm of the sediment of Norsminde Fjord estuary, Denmark. A combination of C ₂ H ₂ inhibition and ¹⁵ N isotope techniques was used in intact sediment cores in short-term incubations (maximum, 4 h). The denitrification capacity exhibited two maxima, one in the spring and one in the fall, whereas the capacity for NO ₃ ⁻ ammonification was maximal in the late summer, when sediments were progressively reduced. The denitrification capacity was always highest in the uppermost 1 cm of the sediment and declined with depth. The NO ₃ ⁻ ammonification was usually higher with depth, but the maximum activity in late summer was observed within the upper 1 cm. The capacity for NO ₃ ⁻ incorporation into organic material was investigated on two occasions in intact sediment cores and accounted for less than 5% of the total NO ₃ ⁻ reduction. Denitrification accounted for between 13 and 51% of the total NO ₃ ⁻ reduction, and NH ₄ ⁺ production accounted for between 4 and 21%, depending on initial rates during the time courses. Changes of the rates during the incubation were observed in the late summer, which reflected synthesis of denitrifying enzymes. This time lag was eliminated in experiments with mixed sediment because of preincubation with NO ₃ ⁻ and alterations of the near-environmental conditions. The initial rates obtained in intact sediment cores therefore reflect the preexisting enzyme content of the sediment.