The purpose of this study was to investigate the effect of starch sources with different degradation rates on ruminal fermentations of lactating dairy cows. Four primiparous lactating Holstein cows were used in a 4 x 4 Latin square design and fed with diets containing different proportions of corn and wheat: diet 1 containing 26.20% of corn; diet 2 containing 16.10% of corn and 7.09% of wheat; diet 3 containing 16.51% wheat and 7.26% corn; or diet 4 containing 25.88%. Dry matter intake values were unaffected by diets. The daily mean ruminal pH decreased from 6.45 to 5.83 (P < 0.05) and minimum pH decreased from 5.90 to 5.29 (P < 0.05) when corn was replaced by wheat in the diet. In the day, time spent below pH 5.8 increased from 0.25 to 13.25 h when corn replaced wheat (P < 0.05). No difference was observed among the total volatile fatty acid values of the experimental groups. The amount of acetate (P < 0.01) and acetate/propionate ratio (P < 0.05) decreased and the propionate level increased (P < 0.05) when wheat was used rather than corn. Ammoniac production remained among similar values in all groups. According to the results of our study, we suggest that using wheat rather than corn in dairy cattle nutrition can increase the risk of subacute ruminal acidosis. This could be related to differences in ruminal degradation rate of starch, that is a major factor in depressing ruminal pH.