Dairy calves commonly have a rumen pH below 5.5 and it is unknown if this is problematic or normal for rumen development. The objective was to test if oral administration of sodium bicarbonate (NaHCO₃) to young calves can affect ruminal: pH, lactate concentration, and populations of select ruminal bacteria. We hypothesized that administration of NaHCO₃ would increase rumen pH, decrease lactate concentration, and affect populations of lactate-producing (Lactobacillus spp.) and lactate-utilizing (Megasphaera elsdenii) bacteria. Twelve Holstein bulls (40.2 ± 1.6 kg BW) were randomized into treatments, arranged factorially. The factorials were treatment and route of treatment. The treatments were: CON-drench (water), BICARB-drench (1M NaHCO₃ in water), CON-bolus (gelatin capsule), BICARB-bolus (NaHCO₃ gelatin capsule). NaHCO₃ was administered twice daily, adjusted weekly, and reached a maximum of 48 g/d. Calves were fed 543g DM/d of a 22% CP, 20% fat milk replacer and 20% CP starter. Rumen contents were sampled at wk 1 and 8 for pH, lactate, and bacteria measurement. Rumen pH did not depend on treatment or route, but was different by week (wk 1, pH = 6.00 ± 0.15; wk 8 pH = 5.19 ± 0.14; P = 0.007). Overall D- and L- lactate concentrations did not differ (7.45 ± 2.15 mM and 7.33 ± 1.98 mM, respectively); although, concentrations of each were numerically higher at wk 8 as opposed to wk 1. Populations of Lactobacillus spp. and M. elsdenii were calculated as a percentage of total 16S rDNA gene copies. Lactobacillus spp. abundance was unaffected by treatments or their interactions, but abundance of M. elsdenii was affected by the interaction of treatment and week (wk 1 CON = 0.05%, wk 1 BICARB = 0.00%, wk 8 CON = 0.19%, wk 8 BICARB = 0.26% of total bacteria, respectively; P = 0.05). Administration of NaHCO₃ by one of 2 oral routes did not affect rumen pH, lactate, or Lactobacillus spp. numbers, but in combination with time, did have an effect on M. elsdenii abundance. This suggests that M. elsdenii was able utilize lactate as a substrate and can withstand low rumen pH. These results support the idea that low rumen pH may be part of the normal rumen development of a dairy calf.