This study examined the effect of the weaning transition and supplemental sodium butyrate—a primary stimulator of rumen development—on rumen fermentation and volatile fatty acid transporter (VFA) abundance. Holstein bull calves (n = 36; age = 10.7 ± 4.1d) were assigned to 1 of 4 treatment groups: 2 pre-weaning groups, animals fed either milk only (PRE-M) or milk, calf starter, and hay (PRE-S); and 2 post-weaning groups: animals fed milk, calf starter, and hay either without supplementation (POST-S) or with 1% wt/wt supplemental butyrate during the weaning transition (POST-B). Milk was provided at 1200 g/d; starter, water, and hay were provided ad libitum. Weaning transition occurred in POST-S and POST-B by reducing milk replacer to 800 g/d in wk 7 and 400 g/d in wk 8, 0 g/d at wk 9 and harvest at wk 10. Rumen pH was measured continuously for 7 d before harvest. At harvest, rumen fluid was analyzed for VFA and rumen tissue was analyzed for VFA transporters. Data were analyzed in SAS with fixed effect of treatment and, where appropriate, repeated effect of week. Between PRE-M and PRE-S, total VFA concentrations increased (11.8 ± 5.8 vs. 35.6 ± 5.6 mM, P < 0.01), mean rumen pH was unaffected (6.16 ± 0.83 vs. 7.44 ± 0.79, P = 0.28), and MCT1 expression was unaffected (6.70 ± 135 vs. 8.39 ± 143 × 10⁵ A.U., respectively, P = 0.30). Between PRE-S and POST-S, total VFA concentrations increased (35.6 ± 5.6 vs. 154.3 ± 15.0 mM, P < 0.01), but mean rumen pH was unaffected (7.44 ± 0.79 vs. 6.39 ± 0.19, respectively; P = 0.48), as was MCT1 expression (8.39 ± 1.43 vs. 7.28 ± 1.35 A.U., respectively; P = 0.58). Between POST-S and POST-B, total VFA concentrations were unaffected (154 ± 15 vs. 131 ± 16 mM, P = 0.23), and mean rumen pH decreased (6.39 ± 0.19 vs. 5.83 ± 0.18, P = 0.05), while MCT1 expression was unaffected (7.28 ± 1.35 vs. 8.17 ± 1.59 × 10⁵ A.U., respectively; P = 0.61). Expression of MCT1 was unaffected by changes in calf starter intake or rumen pH and is not correlated with average daily gain during the weaning transition. These data suggest improvements in nutrient transport may be driven by other transporters or mechanisms.