Prolactin (PRL), along with other hormones, plays a role in oocyte maturation, fertilization, and early embryonic development in mammals. In order to investigate the role of PRL on in vitro oocyte maturation from early follicular growth stages, as well as on fertilization and early embryonic development, we cultured preantral mouse follicles with and without PRL, followed by fertilization of the in vitro matured oocytes. Prolactin significantly improved the rate of oocyte maturation, fertilization, and early embryo development. Four isoforms of PRL-Receptor (R) have been found in whole ovaries of mice: one long (PRL-RL) and three short (-RS(1), -RS(2), and -RS(3)). We examined expression of the four PRL-R isoforms in preantral follicles, in cumulus-oocyte complexes (COCs) and in germinal vesicle GV stage oocytes by RT-PCR. Prolactin-RL, -RS(2) and -RS(3) mRNA, but not -RS(1), were expressed in preantral follicles, COCs, and GV stage oocytes. Our results indicate the prolactin pathway is functional in early preantral follicles, in COCs and in GV stage oocytes, and promotes oocyte maturation, meiosis, fertilization, and early embryonic development.