A review of authors' and published data on maturation of mammalian oocytes in vitro. The following problems are considered: conditions of in vitro oocyte maturation, morphological changes of oocytes during maturation, duration and synchrony of oocyte maturation, RNA and protein synthesis during oocyte maturation, fertilization of oocytes matured in vitro. The oocytes matured in vivo and vitro were shown not to differ morphologically but their ability of subsequent development differed even if meiosis proceeded in the normal way. Upon the resumption of meiosis after the effect of LH in vivo, there appear to occur some substantial changes in the oocytes which are absent during their in vitro maturation.