Mitochondrial inheritance, in contrast to the nuclear inheritance, is only maternal. In order to evaluate the role of mitochondria in early embryonic development and the potential impact of cytoplasmic inheritance on genetic improvement of livestock species, a series of studies were undertaken using the rat as the experimental model. Two types (A and B) of mitochondrial DNA (mtDNA) were identified in the rats used in this study, and a rat colony was developed which contained animals with both mtDNA types. In experiment 1, for each of the 12 rats used, the liver and testicular mitochondrial preparations shared the same type of mtDNA, suggesting that mtDNA is not altered during differentiation of the male gonad. Using offspring from parents of known mtDNA type, we confirmed that they only contain the mtDNA type of the mother, suggesting that either the paternal mitochondria are not transmitted to the offspring or that the mtDNA from the spermatozoal mitochondria is diluted to levels below detection. Differences in mitochondrial morphology were found in rate somatic and germinal cells;In experiment 2, rat embryos were transferred to recipients with the same or different type of mtDNA. The mtDNA of the recipient did not affect the number of females that remained pregnant to term (pregnancy rate) or the number of offspring born. Embryo survival to weaning day was affected (P 0.05). The total number of embryos recovered from the donors and the stage of embryonic development were not affected (P > 0.05) by the type of mtDNA. Similarly, the type of mtDNA did not affect the length of pregnancy, the number of pups born, the numbers of pups weaned, and the weight of male pups at weaning day providing additional evidence to conclude that a difference of mtDNA between embryo and recipient is not a factor to be considered when embryo transfers are to be performed. On this basis, the significantly greater weight at weaning for female pups may not reflect a true biological effect caused by differences on the mtDNA type.