Oocytes were exposed to GnRHa to induce their maturation both in vivo, by administration of the hormone to hypophysectomized rats, and in vitro, in cultures of intact ovarian follicles. Mature oocytes obtained under both these conditions were then exposed in vitro to a sperm suspension for fertilization. Fertilization of control groups of oocytes, isolated from intact or hypophysectomized PMSG-primed hCG-induced ovulators, was 88.3 +/- 3.3% (n = 331) and 90.0 +/- 2.8% (n = 427), respectively, as compared to 82.8 +/- 3.2% (n = 413) for oocytes isolated from hypophysectomized PMSG-primed GnRHa-induced ovulators. Fertilization rate in oocytes treated by GnRHa in vitro was 78.5 +/- 3.1% (n = 247) as compared to 79.3 +/- 4.1% (n = 261) in LH-treated oocytes. These results demonstrate that fertilizability of oocytes undergoing maturation in response to GnRHa is similar to that of oocytes induced to mature by LH. No differences could be detected in the proportions of abnormal oocytes (polyspermic, fragmented and dead) and the zygotes obtained after fertilization of GnRHa- or LH-treated oocytes showed similar ability to cleave.