Fixed ammonium nitrogen is the dominant inorganic form of nitrogen in the sediments of Lake Ontario. The fixed ammonium nitrogen concentration is around 300 µg/g of sediment at the top of a 10‐m core and increases gradually to 525 µg at 150 cm, below which it remains constant. As nitrification is precluded in the reduced sediments below 3 cm, the ammonium ion is either fixed within the sediment clay lattices or migrates upward in the sediment interstitial waters. The uniform fixed and exchangeable ammonium concentrations below 150 cm in the core indicate that the sediment is saturated with respect to ammonium fixation, and the decrease in these concentrations above 150 cm in the core suggest that equilibrium is not attained with the ammonium ion. The deeper sediments probably do not regenerate nitrogen to Lake Ontario, but most of the nitrogen released to the hypolimnion by the sediments is from nitrification, denitrification, and ammonification reactions at the sediment‐water interface. A minimum of 20% of the organic nitrogen input to the sediments is regenerated to the lake from the top 6 cm of sediments. About 90% of the nitrogen in the surface muds is organic: 28–46% as amino acid‐N, 4–7% as hexosamine‐N, and 21–31% as hydrolyzable unidentified‐N. From 29–57% of the total nitrogen could not be accounted for as amino acids, hexosamines, fixed and exchangeable amnonia, nitrate, and nitrite in the surface sediments.