Lithium carbonate is used as a standard treatment for manic depression. While researchers have investigated the teratogenic effects of the lithium carbonate on embryos of various animals in later stages of development, very limited work has been done on the probability of effects at early stages of development. In this study, the teratogenic effect of lithium carbonate was investigated on the early development of Balb/C mouse embryos. A therapeutic dose (i.e. 300 mg/ kg b.w.) was injected into mice intra-peritoneally on days 3.5, 4.5, 5.5 and 6.5 of pregnancy. Then, on day 15.5 of gestation, embryos were collected from the pregnant animals. Among the embryos, 69.7% were healthy, 15.4% resorbed, 4% showed lordosis, 7.3% were undeveloped and 7.9% had eye malformations. A significant increase (P<0.05) in the number of hepatic megakaryocytes and nucleated red cells were also observed among experimental embryos. Analysis of maternal serum proteins prepared from dissected animals showed a significant increase or decrease (P<0.05) in the levels of serum proteins albumin, α2 globulin, β globulin and γ globulin. This research on early developmental stages has shown that pregnant mothers need to be aware of possible teratogenic effects at early stages of pregnancy, though it has been thought that the egg envelope can prevent teratogens from entering. In this case, mothers may need to stop lithium carbonate treatment before they make a decision to be pregnant.