Ruminal fluid was obtained over a 4-yr period from cattle on various diets and supplements to determine the effects of different inocula on the microbial degradation of 3-nitropropanol (NPOH), a toxic metabolite in certain Astragalus spp. (Leguminosae). Nitrite (NO2-) metabolism was also studied in vitro because rapid NO2- reduction is required for the overall detoxification of NPOH. Intra-ruminal supplements of sulfite were ineffective and produced toxic signs in treated animals. Ruminal fluid from cattle on fresh pasture diets enhanced the in vitro metabolism of NO2-, but rates of NPOH disappearance were not significantly affected. Rates of NPOH degradation increased when orchardgrass pasturage was supplemented with molasses. Enhancement of NPOH degradation was achieved with supplements of nitroethane given intra-ruminally at 6.5 or 10 mg/kg body weight. The effect of nitroethane on NO2- reduction was not always observed, but the NO2- rates of metabolism always exceeded those of NPOH. The rate of NPOH degradation also increased when nitroethane was added to a molasses supplement. However, the volatility of nitroethane under field conditions prompted a search for a more stable inducer and the sodium salt of nitroethane was subsequently evaluated. The salt of nitroethane, given intra-ruminally at 20 mg nitroethane/kg body weight, resulted in the highest rate of NPOH degradation; this was similar to that reported for 3-nitropropionic acid, a nitroalkane that is much less toxic to ruminants than NPOH.