The relationship between dietary variations and oxidative stress has been poorly investigated under field conditions. Changes of the diet starch content were introduced to study whether they could induce oxidative stress in early lactating heifers. In addition, the association between blood metabolic, endocrine responses and indicators of milk cellular immune functions have been investigated. Ten heifers were randomly assigned to two groups and fed with a basal total mixed ration until 34 days in milk (DIM). At 35 DIM control group continued to receive the same ration and the other group was allotted to experimental diets with low and high starch contents. The results of this study show that even a sudden change of diet is not associated with a consistent and significant change in most of the metabolic parameters considered. However, insulin and growth hormon (GH) levels were associated to a decreased probability of low leukocyte and neutrophil counts, but also to an increase in blood NAGase activity and milk lysozyme activity. These results suggest that GH induction and insulin release could have a selective influence on both number and activity of immune bells in blood and in milk.