Sodium acetate (NaAcet) treatment increases milk fat yield when ruminally infused or mixed in a TMR. Sodium acetate increases acetate supply, but also increases dietary cation anion difference (DCAD), which can also increase milk fat yield. Our objective was to determine if the effect of NaAcet on milk fat production is due to increasing acetate supply or DCAD. The study included 12 multiparous cows in a replicated 3x3 Latin square balanced for carryover effects with 14 d periods. Treatments were control TMR (17% CP, 30% NDF, 26% starch, and 5% EE on DM basis), NaAcet providing 590 g/d of acetate, and sodium bicarbonate (NaHCO₃) providing an equal amount of sodium as the acetate treatment. All treatments were mixed in the control TMR and fed ad libitum once a day and intake was recorded daily. Milk production was recorded daily with an integrated milk meter and milk components and milk fatty acid profile were determined the last 3 d of each period. Blood and fecal samples were collected every 9 h during the last 3 d of each period. Plasma acetate was determined by GC/MS. Production data were analyzed in JMP Pro 13 using a model that included the random effect of cow and period and fixed effect of treatment and a protected LSD mean separation. Plasma metabolites were analyzed with proc mixed in SAS 9.4 with repeated measures. Milk yield was not different between treatments, but NaAcet and NaHCO₃ increased dry matter intake by 6.6 and 7%, respectively (P = 0.02). Milk fat yield was increased 134 g by NaAcet and 118 g by NaHCO₃ (P < 0.01). Sodium acetate increased milk fat yield predominantly by increasing the yield of de novo and mixed source fatty acids, while NaHCO₃ increased the yield of preformed and de novo fatty acids. Sodium acetate increased plasma acetate and decreased plasma glucose concentration during the afternoon and early evening, which coincides with the higher intake period of the day. In conclusion, NaAcet and NaHCO₃ both increase milk fat production, but NaAcet achieves this through increased mammary gland de novo lipogenesis.