Samples of sediment from Lake St. George, Ontario, Canada, were incubated in the laboratory under an initially aerobic gas phase and under anaerobic conditions. In the absence of added nitrate (NO(3)) there was O(2)-dependent production of nitrous oxide (N(2)O), which was inhibited by acetylene (C(2)H(2)) and by nitrapyrin, suggesting that coupled nitrification-denitrification was responsible. Denitrification of added NO(3) was almost as rapid under an aerobic gas phase as under anaerobic conditions. The N(2)O that accumulated persisted in the presence of 0.4 atm of C(2)H(2), but was gradually reduced by some sediment samples at lower C(2)H(2) concentrations. Low rates of C(2)H(2) reduction were observed in the dark, were maximal at 0.2 atm of C(2)H(2), and were decreased in the presence of O(2), NO(3), or both. High rates of light-dependent C(2)H(2) reduction occurred under anaerobic conditions. Predictably, methane (CH(4)) production, which occurred only under anaerobiosis, was delayed by added NO(3) and inhibited by C(2)H(2). Consumption of added CH(4) occurred only under aerobic conditions and was inhibited by C(2)H(2).