In order to elucidate the mechanism of the inhibitory effect of nitrate and its denitrification intermediates nitrite, NO and N P O on methanogenesis in anoxic environments, we tested possible toxic effects of these N-compounds on the methanogenic bacteria Methanosarcina barkeri and Methanobacterium bryantii which are ubiquitous in methanogenic rice field soils. The different N-compounds inhibited H P -dependent methanogenesis by these bacteria to different extents. Nitrate showed the weakest inhibition of methanogenesis in both bacteria, followed by N P O and nitrite for Ms. barkeri, and nitrite and N P O for Mb. bryantii, respectively. In both bacteria, the strongest inhibition was caused by NO. Concentrations of 30 mM nitrate still enabled a CH R production rate of 25^40% of that before the addition of the N-compound, whereas NO completely inhibited methanogenesis at concentrations v0.8^1.7 WM (equivalent to 50^100 Pa NO partial pressure). Removal of NO by replacing the atmosphere with H P /CO P (8:2) resulted in resumption of methanogenesis only if the bacteria had been treated with NO concentrations 90.8 WM (50 Pa). Removal of N P O from the cultures resulted in resumption of methanogenesis if Mb. bryantii had been treated with 995 WM N P O (500 Pa) or Ms. barkeri with 9950 WM N P O (5 kPa). These results show that the denitrification products of nitrate can inhibit CH R production both reversibly and irreversibly depending on the type of methanogenic bacterium and the applied concentration of the N-compound. In a separate experiment with methanogenic rice field slurries addition of nitrate resulted in immediate inhibition of CH R production. Nitrate was consumed resulting in the sequential accumulation of nitrite, NO and N P O which were subsequently utilized. Nitrite and N P O reached maximum concentrations that would have been inhibitory in the methanogenic bacterial cultures examined.