AbstractMirex is a pesticide that is environmentally stable, accumulates in body tissues, and is embryo‐ and feto‐toxic at high concentrations in vivo. This study is the first to evaluate the effects of mirex on organogenesis‐stage embryos in vitro. Mouse embryos were exposed on gestation day 8.5 for 24 h in whole‐embryo culture to mirex at 100, 200, or 400 µg/ml dissolved in xylene and compared with xylene‐treated controls (1, 2, or 4 µl/ml, respectively) and untreated controls. Embryos were evaluated for malformations, somite number, total protein content, and visceral yolk sac circulation. Potential embryotoxic mechanisms were evaluated by using PCNA stain for cell proliferation and the TUNEL assay for apoptotic cell death. Mirex‐exposed embryos demonstrated increased malformation rates and decreased total embryonic protein contents at ≥200 µg/ml mirex, and decreased somite numbers and VYS circulation at ≥100 µg/ml mirex, compared with xylene‐treated controls. There was no difference in PCNA levels or TUNEL staining in mirex‐treated embryos compared with xylene‐treated controls or untreated controls. Thus, mirex is embryotoxic in vitro to early organogenesis stage mouse embryos at concentrations ≥100 µg/ml, but the effects do not appear to be mediated by changes in cell proliferation or apoptotic cell death. © 2002 Wiley‐Liss, Inc.