Studies of the effect of gonadotrophins on the cumulus and oocyte or on the whole follicle in vitro offer a good tool for understanding the mechanisms of oocyte maturation, the ability of follicles of different sizes to respond to gonadotrophins and the processes of atresia. The birth of normal young from intrafollicular oocytes matured in vitro furnishes proof that in-vitro techniques are valuable as long as follicular integrity is maintained during the whole culture period.