The purpose of this study was to determine the adverse effects of methylmercuric chloride(MMC) against the fetal growth and the ossification rate of fetal pectoral and pelvic girdle, stermebrae, ribs and tail in pregnant Fischer 344 rats administered orally on day 7 of gestation. The resulted obtained are as follows. The weight and size of fetus were highly reduced by MMC. The reduction of fetal weight and size were 16. 2%~24.5%(p<0.01), and 34.1%~48.8%(p<0.01), and that of the litter’s weight were 67.0%(p<0.01) and 89.2%(p<0.01) by 20 and 30mg/kg MMC, respectively. Ossification centers were never formed in pectoral and pelvic phalanges and sternebrae, and was reduced as much as 70% in tail by 30mg/kg MMC. And also those were 82.4%~ 91.2%(p<0.01) in ischium, and 52.4~66.7%(p<0.01) in the others(ilium, fenur, tibia, fibula, metatarsals)of pelvic girdle by 30 mg/kg MMC. Ossification of sternebrae was terrible. 5th bone of sternebrae was not ossificated by 20 and 30 mg/kg MMC(p<0.01), and 2nd was also not ossificated by 30 mg/kg MMC(p<0.01).And reduction of ossification rate was 84.8~97.8%(p<0.01) in the others of sternebrae by 30 mg/kg MMC. And then, the reduction of ossification rate was 26.65~49.8%(p<0.01) in fetal ribs by 30 mg/kg MMC, and they were trend to increased as following from center to each edge. In conclusion, it was observed that fetal weight, size, and ossification of each bone were highly significantly reduced by the increased dosage of MMC.