Abstract : Cumulus-oocyte complexes (COCs) were recovered from ovaries of bitches during anoestrus. The ultrastructural organisation of COCs was determined before and after 72 h in vitro maturation (IVM) by transmission electron microscopy. The aim of the study was to determine the quality of oocytes used for IVM and to assess cytoplasmic maturation of IVM metaphase (M) II oocytes. In addition, we examined whether the oocytes that did not reach MII were engaged in an erratic maturation process or whether they were blocked during their progression through a normal maturation process. Before IVM, there were two populations of oocytes: (1) oocytes with a centrally located germinal vesicle, a transcriptionally active aspect and an immature cytoplasm; and (2) oocytes with an eccentric nucleus, a transcriptionally inactive aspect and a more mature cytoplasm. After IVM, most oocytes were still at the germinal vesicle stage with three different patterns and all showing a good synchronisation between nuclear and cytoplasmic maturation. MI oocytes had a similar cytoplasmic maturation to that observed in vivo, but failed to complete meiosis; however, IVM MII oocytes had a very poor cytoplasmic maturation. Ultrastructural analysis demonstrated that even when nuclear maturation is achieved, cytoplasmic maturation may not be obtained in vitro. Thus, all IVM systems should be evaluated on both criteria.