The effect of nitrate, nitrite, nitric oxide (NO), and nitrous oxide on a mixed, mesophilic (35 degrees C) methanogenic culture was investigated. Short-term inhibition assays were conducted at a concentration range of 10-350 mg N/L nitrate, 17-500 mg N/L nitrite, 0.02-0.8 mg N/L aqueous NO, and 19-191 mg N/L aqueous nitrous oxide. Simultaneous methane production and N-oxide reduction was observed in 10 and 30 mg N/L nitrate and 0.02 mg N/L aqueous NO-amended cultures. However, addition of N-oxide resulted. in immediate cessation of methanogenesis in all other cultures. Methanogenesis completely recovered subsequent to the complete reduction of N-oxides to nitrogen gas in all N-oxide-amended cultures, with the exception of the 500 mg N/L nitrite- and 0.8 mg N/L aqueous NO-amended cultures. Partial recovery of methanogenesis was observed in the 500 mg N/L nitrite-amended culture in contrast to complete inhibition of methanogenesis in the 0.8 mg N/L aqueous NO-amended culture. Accumulation of volatile fatty acids was observed in both cultures at the end of the incubation period. Among all N-oxides, NO exerted the most and nitrate exerted the least inhibitory effect on the fermentative/methanogenic consortia, The effect of multiple additions of nitrate (300 mg N/L) on the same methanogenic culture was also investigated. Long-term exposure of the methanogenic culture to nitrate resulted in an increase of N-oxide reduction rates and decrease of methane production rates, which was attributed to changes in the microbial community structure due to nitrate addition.