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Notes: Abstract The ammonification rates in surficial sediments of the Marica lagoon (Rio de Janeiro, Brazil) were estimated using two methods: (1) A direct evaluation of the ammonia productivity in sediments by means of an incubation procedure: the ammonia production rate is deduced from the difference in the ammonia concentrations in interstitial waters which were extracted directly after sampling and after incubation from two samples collected at the same point. (2) Since ammonia results essentially from the destruction of organic nitrogen, the ammonia productivity will be obtained by evaluating the nitrogen destruction rate: the amount of organic N mineralized is assumed to correspond to NH inf4 sup+ produced. The organic N destruction rate can be calculated by means of the matter balance equation provided the organic nitrogen profile and the sediment accumulation rate are known. The sedimentation rate, evaluated by means of the 210Pb method, differed in various areas of the lagoon. It was 0.35 cm y−1 in the open area of the lagoon (at 1.6 m in water depth), near the mouth of the Mombuca river (site M2); and 0.27 to 0.28 cm y−1 at 1.6 to 1.7 m at a greater distance of the river mouth (site M4). In a confined part of the lagoon (Z1), surrounded by reeds (Typha dominguensis), the rate is lower: 1 to 1.4 mm y−1 in the upper level (from 0 to 4 cm) where the organic matter amount reaches 60 to 80% (dry w.), and 1.1 to 1.2 mm y−1 deeper (below 5 cm) where organic matter corresponds to less than 40% of the sediment. The organic nitrogen destruction rate was calculated to be 0.8 · 10−6 µmol cm−3 s−1 (= 0.07 µmol cm−3 d−1) for the upper sediment level (0–2 cm) of site M4. This computed rate is slightly less than the value 1.1 · 10−6 µmol NH inf4 sup+ cm−3 s−1 obtained by means of the incubation experiment. For the upper level of the site close to the reeds (Z1), the calculated nitrogen destruction rate was 1.7 · 10−6 µmol cm−3 s−1 (0.15 µmol cm−3 d−1), the ammonification rate obtained by the incubation method 1.4 · 10−6 µmol cm−3 s−1. It appears that the two methods provided similar results.