This study aimed to evaluate the methane-reducing potential of individual and combined treatments of low levels of nitrate (NIT) and a mixture of capric/lauric acid (CL) in dairy cows. Both in vitro and in vivo experiments were conducted. In the in vitro experiment, the anti-methanogenic effects of NIT (1.825 mmol/l) and CL (250 mg/l; capric acid, 125 mg/l + lauric acid, 125 mg/l) were evaluated in a 2 x 2 factorial design using consecutive batch incubations with rumen fluid. The NIT and CL reduced (P < 0.05) methane production by 9.2% and by 21.3%, respectively. However, combining NIT with CL did not show (P > 0.05) any benefit in methane reduction compared to the use of CL alone. In in vivo experiment, eight multiparous dry Holstein cows were fed two diets in a crossover design for two 21-day periods (14 days of adaptation and 7 days of sampling). The treatments were: 1) silage-based basal diet + 100 g stearic acid per cow/d (CON) and 2) silage-based basal diet + 50 g capric acid + 50 g lauric acid per cow/d (CL). Gas emissions were measured using open-circuit respiration chambers. Methane production (g/d) was reduced (by 11.5%; P = 0.012) when the diet was supplemented with CL. However, supplementation with CL increased ruminal ammonia-N concentration (by 28.5%; P = 0.015) and gas ammonia production (g/d; by 37.2%; P = 0.005). Ruminal pH, protozoa count, and total and individual volatile fatty acid concentrations (VFA) did not differ (P > 0.05) between the treatments. Treatment did not affect the intake and total tract apparent digestibility (P > 0.05). In conclusion, our results suggest that low CL levels have anti-methanogenic potential. However, low levels of CL may compromise nitrogen use efficiency.