Abstract: The objective of this study was to investigate the effect of a high-grain (HG) diet on the ruminal epithelial structure and the transcript levels of genes encoding monocarboxylate transporters (MCTs) and a Na⁺/K⁺-ATPase in goats. Ten ruminally fistulated, castrated male goats were randomly assigned to either a hay diet (Hay,0% grain, n=5) or a HG diet (HG,65% grain, n=5) with continuous feeding for 7 weeks. Each week, at 0, 2, 3, 4, 6, 8 and 12 h after morning feeding, rumen fluid was collected to monitor the changes in ruminal pH. The rumen fluid collected at 0, 3, 6, and 12 h after morning feeding was used to determine the volatile fatty acid (VFA) concentration. The results showed that, compared with the control, the HG diet significantly reduced rumen pH, acetate concentration, and the acetate-propionate ratio (P<0.001), and significantly increased the concentrations of ruminal propionate, butyrate, and other VFAs (P<0.001). The HG diet significantly increased the length but decreased the width of rumen papillae. The surface of the rumen papillae did not differ significantly between the two groups. The HG diet caused substantial damage to ruminal epithelial cell mitochondria. Compared with the hay-fed group, the HG-fed group showed increased transcript levels of genes encoding MCT1 (P<0.001) and the Na⁺/K⁺-ATPase (P=0.001), but decreased transcript levels of the gene encoding MCT4 (P=0.041). There was no significant difference in the transcript levels of MCT2 between the hay-fed and HG-fed groups (P=0.305). Correlation analyses revealed that the transcript levels of the genes encoding MCT1 and Na⁺/K⁺-ATPase were negatively correlated with ruminal pH and acetate concentration, but positively correlated with total VFA, propionate, and butyrate concentrations. The MCT4 mRNA level was positively correlated with pH, but negatively correlated with total VFA, propionate, and butyrate concentrations. These results suggested that the changes in ruminal pH and VFA concentrations were associated with the changes in the expression levels of MCTs and Na⁺/K⁺-ATPase. These findings provide important information for understanding HG diet-induced ruminal epithelial function disorder in ruminants.