Nitrate is known to suppress CH4 production in anoxic soil. Three hypotheses have been proposed to explain the mechanism: redox shift; competition for methanogenic substrates by denitrifying bacteria; or inhibition by toxic denitrification intermediates (nitrite, NO, N2O). Since recent studies have shown that methanogens and methanogenesis are not necessarily prevented in being active under positive redox potential, only the latter two hypotheses remain valid. In order to test which of the remaining two hypotheses better explains this suppression, we studied the effect of added electron donors (acetate, H2, propionate) on the suppression of CH4 production in rice soil. Addition of nitrate to a methanogenic rice soil slurry completely suppressed CH4 production. The addition of acetate, H2 or propionate in the absence of nitrate stimulated CH4 production. However, when they were added together with nitrate, CH4 production was also suppressed, although the complete suppression of methanogenesis was shortened. Thus, addition of electron donors could not prevent the inhibition of methanogenesis by nitrate. We conclude that the main mechanism involved in the suppression of CH4 production by nitrate is the inhibition of methanogenesis by denitrification intermediates rather than the competition between denitrifiers and methanogens for substrates. The duration of the suppression by nitrate was closely related to the ratio of electron donor to electron acceptor. The greater this ratio, the shorter the suppression, indicating that nitrate and its potentially toxic denitrification intermediates nitrite, NO and N2O were then faster reduced to non-toxic N2.