In this study, the effect of preovulatory treatment with RU486, for different lengths of time and combinations of days, was shown in terms of the ova recovery, in vitro fertilization of recovered ova, in vivo fertilization and quality of fertilized ova in PMSG/hCG-primed mice. Female mice were injected with PMSG followed 48 h later by hCG to induce superovulation. Mice received RU486 (20 mg kg-1 body wt) for 1, 2, 3 and 4 preovulatory days (in different combinations). Ovulation, as judged by the number of ova recovered at 14 to 14.5 h post-hCG, was depressed (P < 0.001), and the total number of embryos recovered at 40 h post-hCG was low (P < 0.001), in mice receiving a minimum of two consecutive days' treatment (day before PMSG + day of PMSG; or day before hCG + day of hCG) of RU486 under study. Quality of ova recovered from RU486-treated animals was not affected as determined by their ability to become fertilized in vitro. In vivo fertilization, as determined by the recovery of 2-cell embryos, was suppressed significantly in mice treated with RU486 for four consecutive preovulatory days (P < 0.001). A varied degree of premature compaction was observed in 2-cell embryos immediately upon retrieval from the oviduct of RU486-treated animals, the effect being most marked in mice receiving RU486 for a minimum of two consecutive preovulatory days under study. It is suggested that premature compaction of early embryos was under the continuous influence of the luminal environment of treated animals and might be the reason for their degeneration at later stages in the reproductive tract and for a low pregnancy rate as shown by other studies. Compacted embryos decompacted within 15-30 min in vitro and led to normal blastocyst formation in vitro in RU486-free culture medium.