Placenta is a privileged organ which has a storage function during pregnancy. Thus, it establishes an interface between maternal circulation and fetus. Therefore, any deviation from normal placental development may constitute a potential threat to fetus growth. In this work, we try to study the subcellular behavior as well as the effects of lanthanum (La), on placenta of gestating rats after its intraperitoneal injection under soluble solution. Using TEM, the ultrastructural observations of placenta tissues from lanthanum-treated rats showed many electron-dense surcharges in the lysosomes of maternal connective tissues from maternal part of placenta as well as syncytiotrophoblast and cytotrophoblast from fetus side of placenta respectively. At a concentration of 48 mg/kg, lanthanum was found to cause change in nucleus contouring, vacuolations, expansion of the rough endoplasmic reticulum, mitochondrial sufferance and cytoplasmic rarefication. These modifications covered both side of placenta. The ultrastructural study of control rats showed a normal aspect of the ultrastructure of the two parts of placenta. Finally, our data strongly suggests the transplacental passage of lanthanum and its accumulation within the lysosomes of both side of placenta which may constitute a potential threat to fetus growth and development. We try through this work to study the cellular distribution of lanthanum after its intraperitoneal administration as a soluble solution to attempt to characterize its eventual transfer across the placenta from mother to the fetus.