In vitro growth systems of preantral follicles allow studying the effect of various endocrine, paracrine, and autocrine factors on follicular growth and oocyte maturation. CRH is a 41-amino-acid neuropeptide responsible for endocrine, autonomic, immunological, and behavioral responses of mammals to stress and has two receptors, CRH receptor type 1 (CRH-R1) and CRH-R2. Antalarmin, a CRH-R1 antagonist, has been used to elucidate the role of CRH in stress, inflammation, and reproduction. The present study describes in vitro growth of mouse preantral follicles, early embryo development, and steroidogenesis in the presence of CRH and its antagonist antalarmin. We cultured 732 follicles in control media, 1306 in CRH 10(-7) mol/liter, and 1202 in CRH 10(-7) plus antalarmin 10(-6) mol/liter. The culture medium was assayed on alternate days for 17β-estradiol, progesterone, and β-human chorionic gonadotropin. Total RNA was extracted from preantral follicles as well as early preimplantation embryos and was assessed by real-time RT-PCR for the expression of CRH-R1 and CRH-R2 mRNAs. Hormone analysis showed that the CRH group had lower levels of 17β-estradiol, progesterone, and β-human chorionic gonadotropin as the culture progressed, in comparison with the other two groups. RT-PCR demonstrated the presence of CRH-R1 and CRH-R2 in all stages of preantral follicle culture. Morula/blastocyst-stage embryos expressed only CRH-R1. In conclusion, CRH has an inhibitory effect on in vitro fertilized oocytes, resulting from cultured preantral follicles at all stages of preimplantation embryo development. Furthermore, the presence of CRH in the culture medium inhibits steroidogenesis by preantral mouse follicles cultured in vitro.