3-Nitrooxypropanol (NOP), an enzymatic inhibitor that has consistently reduced methane (CH₄) emissions in sheep, dairy and beef cattle (up to 59% reduction on a long-term basis), and monensin (MON), an ionophore that has a moderate, and sometimes transitory, effect on CH₄ reduction were used in this study. The objective was to evaluate the effect of NOP, MON and the combination of both on CH₄ production when added to a finishing diet (85% barley grain, 10% barley silage, and 5% vitamin-mineral supplement; DM basis) using Rusitec fermenters. Two Rusitec apparatuses each equipped with 8 fermenters were used in a completely randomized block design with 2 blocks (apparatus) and 4 treatments: Control, NOP (2 mg), MON (2 mg) and NOP+MON (2 mg+2 mg). Within each apparatus, 2 fermenters were randomly assigned to a treatment. Treatments were supplied daily with 10 g of diet. The experiment included an adaptation period without treatment supplementation (8 d), a treatment period (7 d), and a recovery period where treatments were discontinued (3 d). During the treatment period, DM digestibility was not affected. Total VFA and molar proportion of propionate and butyrate were not affected (P > 0.05) but acetate was reduced (P < 0.01) with addition of NOP (8.3%) and NOP+MON (14.9%). Methane production was reduced (P < 0.01) by 69.4 and 68.2% with NOP and NOP+MON respectively, while H₂ production was increased (P < 0.01) by 76.7 and 75.2% respectively for the same treatments compared with Control. Treatments had no effect on the copy number of the 16S rRNA gene for total bacteria (P > 0.21); however, that for methanogens was reduced (P < 0.01) with NOP and NOP+MON treatments. During the recovery period on d 18, no effect (P > 0.20) was observed for CH₄ and H₂ production; however, a trend for lower acetate concentration with NOP and NOP+MON was still observed (P = 0.08). Treatments including NOP were effective in reducing CH₄ production in vitro using a finishing diet with a concomitant increase in H₂ production; however, the combination of NOP and MON did not lead to greater CH₄ reduction than NOP alone.