The objective of this study was to evaluate the effects of an oral drench with a direct-fed microbial (Megasphaera elsdenii NCIMB 41125; Lactipro Advance, MS Biotec, Wamego, KS) on rumen pH, feed intake, and milk yield of lactating dairy cows. The study was conducted at the University of Kentucky Coldstream Dairy from March to May 2017. Eight late-lactation (238.5 ± 54.7 DIM) Holstein cows were enrolled in a crossover trial divided into 2 experimental periods. Each period lasted 8 d with a 4-wk washout period. Upon enrollment, cows were randomly assigned to either an M. elsdenii (LACT) or distilled water (CON) drench. Drenching occurred on d 1 of the experimental period. On d 5 of the experimental period, cows received a challenge grain mix rich in highly fermentable carbohydrates to induce subacute ruminal acidosis (SARA). Rumen pH was recorded using the smaXtec rumen bolus in 10-min intervals, feed intake was measured using Insentec feed bins, and milk yield was recorded using the AfiLab automatic meter. Daily time and area under the curve (AUC) were calculated using 2 different pH thresholds to define SARA condition, pH below 5.8 or 5.6. A mixed linear model was used to determine the relationships of treatment and rumen pH, feed intake, milk yield, AUC, and time spent in SARA condition during the trial. Throughout the trial, LACT cows had greater overall mean rumen pH (LACT = 6.23 ± 0.01, CON = 6.11 ± 0.01; P < 0.01). In addition, LACT cows consumed more feed on an as-fed basis (LACT = 41.3 ± 2.5, CON = 37.4 ± 2.5 kg/d; P = 0.04) and had greater milk yield (LACT = 28.7 ± 1.6, CON = 26.1 ± 1.6 kg/d; P < 0.01). Cows on LACT drench had lesser AUC below pH 5.8 (LACT = 20.9 ± 27.3, CON = 81.4 ± 27.4; P = 0.01), and AUC below pH 5.6 (LACT = 6.8 ± 13.9, CON = 37.3 ± 13.9; P = 0.03). In addition, LACT cows spent less time below pH 5.8 (LACT = 1.8 ± 1.4, CON = 4.7 ± 1.4 h/d; P < 0.01), and pH 5.6 (LACT = 0.6 ± 0.9, CON = 2.7 ± 0.9 h/d; P = 0.01). Overall, cows drenched with M. elsdenii had greater rumen pH, experienced shorter and less intense SARA, and had greater milk yield.