The objective of this experiment was to study the effects of dietary nitrate (NO₃) and docosahexaenoic acid (DHA; C22:6 n-3) on enteric methane (CH₄) production in lactating dairy cows. Twenty-eight lactating Holstein dairy cows were grouped into 7 blocks of 4 cows each. Within blocks, cows were randomly assigned to one of 4 treatments: CON (urea as alternative NPN source); NO₃ (21 g/kg DM); DHA (3 g/kg DM and urea as alternative NPN source); or NO₃ + DHA (21 g/kg DM and 3 g/kg DM, respectively). Cows were fed a total mixed ration consisting of 20% grass silage, 50% corn silage and 30% concentrate on a DM basis. Treatments were included in the concentrates. Methane production was measured during a 5-d period in climate respiration chambers after adaptation to the diet for 12 d. Previous to this 17-d period, cows assigned to a treatment including NO₃ were gradually pre-adapted to the treatment dose of NO₃ over a period of 21 d. Preliminary results show a significant (P < 0.05) NO₃ × DHA interaction for CH₄ expressed in g/d, and cows produced on average 368, 264, 369 and 298 g CH₄/d on treatments CON, NO₃, DHA and NO₃ + DHA, respectively. This interaction effect is explained by a lower DMI for the NO₃ treatment despite the restricted feeding regimen during the CH₄ measurement period. Per kg DMI, cows receiving NO₃ produced less CH₄ than cows receiving no NO₃ in their diets (17.5 vs. 22.4 g/kg DMI; P < 0.01). Feeding DHA did not affect CH₄ production per kg DMI, but did result in a higher (P < 0.01) CH₄ production per kg FPCM. Milk production was not affected by treatment, but FPCM was reduced (P < 0.05) by DHA because of a reduced milk fat concentration (P < 0.01), which averaged 29.6 and 41.1 g/kg for treatments with or without DHA, respectively. Milk protein concentration was reduced (P < 0.05) by NO₃ and averaged 30.0 and 31.0 g/kg for treatments with or without NO₃, respectively. In conclusion, NO₃ but not DHA reduced enteric CH₄ production and there were no interaction effects on CH₄ production expressed in g/kg DMI and g/kg FPCM.