The study was designed to reveal the role of programmed cell death (PCD, or apoptosis) induced by methylmercury chloride (MMC) in SD rat embryos, wish TdT-mediated dUTP nick end labeling (TUNEL), Nile blue sulfate (NBS) vital stailing electron microscopy (SEM) and in vivo teratogenic methods (TEM). Pregnant rats were intraperitoneally injected at gestation day 9.5 with doses of 0, 0.2, 0.4, 0.8, 1.6 and 3.2 mg/kg MMC and killed at 11.5 day later. Results showed that the PCD of embryonic nervous system was apparently dose dependent with MMC during rat head-fold stage. Observations under SEM and TEM showed that MMC caused pathologic changes of epithelia and organellae in embryonic brain such as atrophied or decreased microvilli cavernous damages and mitochondrial swelling and so on. Though MMC could result in developmental anomalies of embryonic brain and other organs, the main defect was open neural tube. The experiment study suggested that over PCD may be one of the teratogenic mechanisms of MMC on rat embryos, developing brain in particular.