The inhibitory effects of 90-189 mg(-1) of sulfide and 25-75 mg-N l(-)1 of nitrate on methanogenesis were investigated in a mixed methanogenic culture using butyrate as carbon source. In the initial phase of 90 mg l(-1) S(2-) test, autotrophic denitrification of nitrate occurred with sulfide as the electron donor. Then the sulfate-reducing strains converted the produced sulfur back to sulfide via heterotrophic oxidation pathway. Methanogenesis was not markedly inhibited when 90 mg l(-1) of sulfide was dosed alone. 'When 25-75 mg-N l(-1) of nitrate was presented, initiation of methanogenesis was seriously delayed. Nitrogen oxides (NO,), the intermediates for nitrate reduction via denitrification pathway, inhibited methanogenesis. The 90 mgl(-1) of sulfide favored heterotrophic dissimilatory nitrate reduction to ammonia (DNRA) pathway for nitrate reduction. Possible ways of maximizing methane production from an organic carbon-rich wastewater with high levels of sulfide and nitrate were discussed. (C) 2011 Elsevier Ltd. All rights reserved.