Certain morphological changes at the subcellular level caused by the current techniques for in vitro embryo production seem to affect mitochondria. Many of these, including dysfunctional changes, have been associated with the presence of serum in the culture medium. Thus, the aim of the present work was to assess the mitochondrial dynamics occurring in embryos during the first 4 days of development, in order to analyze the most appropriate time for adding the serum. We used transmission electron microscopy (TEM) micrographs to calculate the embryo area occupied by the different morphological types of mitochondria, and analyzed them with Image Pro Plus analyzer. The results showed hooded mitochondria as the most representative type in 1- to 4-day-old embryos. Swollen, on-fusion, orthodox and vacuolated types were also present. When analyzed in embryos cultured without serum, the dynamics of the different mitochondrial types appeared to be similar, a fact that may provide evidence that the developmental changes control the mitochondrial dynamics, and that swollen mitochondria may not be completely inactive. In contrast, in culture medium supplemented with serum from estrous cows, we observed an increased area of hooded mitochondria by developmental day 4, a fact that may indicate an increased production of energy compared with previous days. According to these results, the bovine serum added to the culture medium seems not to be responsible for the functional changes in mitochondria.