The effect of nitrate addition on microbial H2S production in a seawater-flooded oil reservoir model column with crude oil as carbon and energy source was investigated. Injection of 0.5 mM nitrate for 2.5-3.5 months led to complete elimination of H2S (initially 0.45-0.67 mM). The major decline in H2S level coincided with the first complete nitrate consumption and production of nitrite. When nitrate was excluded, H2S production resumed after approximately 2.5 months and reached previous levels after approximately 5 months. Using a fluorescent antibody technique, three populations each of sulfate-reducing bacteria (SRB) and nitrate-reducing bacteria (NRB) were monitored. SRB dominated the anoxic zone prior to nitrate addition, comprising 64-93% of the total bacterial population. The monitored NRB constituted less than 6% and no increase was observed during nitrate addition (indicating that other, unidentified, NRB populations were present). After 1-3 months without significant H2S production (3.5-5.5 months with nitrate), the SRB population collapsed, the fraction being reduced to 9-25%. The dominant SRB strain in the column, which constituted on average 94% of the monitored SRB population, was partly/completely inhibited by 56/75 muM nitrite in batch culture tests. Similar nitrite concentrations (50-150 muM) were detected in the column when the H2S level declined, indicating that nitrite inhibition was the main cause of H2S elimination. The results from this study indicate that nitrate/nitrite can be used to prevent detrimental SRB activity in oil reservoirs.