Laying hens, one year of age, fed a commercial concentrate were killed, and their caecal contents inoculated into a nutrient broth with starch as the energy source. Cultures were incubated for 24 h and production of methane, hydrogen and volatile fatty acids (VFA) measured. Thirteen mols of VFA were produced per 1 mol of methane in control incubations. Acetate, propionate and butyrate accounted for 89-94% of metabolite carbon. Only traces of lactate were produced. Monensin and salinomycin at 5 mg/l significantly decreased production of methane, total VFA, acetate, butyrate and valerate. Similar fermentation shifts were found in cultures containing 2-bromoethanesulphonic acid (2-BES) at 10 mmol/l. Propionate percentages were increased in all treated cultures. Hydrogen recoveries were incomplete in control cultures (70.5-82.1%). The headspace gas of control cultures contained ca 1% (v/v) of molecular hydrogen. Approximately twice more hydrogen gas accumulated in treated cultures.