The production of methane by ruminal bacteria involves the conversion of potentially useful energy-rich substrates into a form that cannot be used by the ruminant host. A possible strategy for reduction of ruminal methane production is to divert the flow of reducing substrates away from methanogenesis into alternative electron sinks. In this study we examine the potential for nitrocompounds to serve as such electron sinks. When 3-nitropropionate (5, 10, or 20 mM) was added to mixed populations of ruminal microbes incubated under a H-2:CO2 (1:1) atmosphere and with added formate, up to 68% less methane was produced and reductant was directed towards increased propionate production. When nitrate (5, 10 or 20 mM) was added to such populations, methane production was inhibited to a lesser degree than with 3-nitropropionate. Addition of cells of a nitropropionate-metabolizing bacterium, strain NPOH1, to mixed ruminal populations did not change the effect of 3-nitropropionate on methane production, although more 3-nitropropionate was metabolized. However the addition of cells of strain NPOH1, which also reduces nitrate, to such populations drastically changed the effect of nitrate on methane production. In the latter case, reductant was directed away from methane biosynthesis to the reduction of nitrate by strain NPOH1, as evidenced by up to an 18-fold decrease in methane production. These results suggest that 3-nitropropionate and nitrate reduce methane production, by different mechanisms.