Prenatal effects of nickel result from direct insults to the mammalian embryo as well as from indirect ones through maternal damage. Nickel may upset the hormonal balance of the mother and can impair the development of the preimplantation embryo. The metal can cross the feto-maternal barrier and enter the fetus. In addition to an increase in prenatal and neonatal mortality, nickel can produce different types of malformations in the surviving embryos but its teratogenic action seems to be delayed, probably as a result of retarded transfer via the placenta. No definite conclusions can be reached, at the present time, as to whether the embryotoxicity and fetal toxicity of nickel is eventually related to its mutagenic properties. Nickel alters macromolecular synthesis but no convincing evidence has been provided of its ability to produce gene mutations or structural chromosome aberrations in mammalian cells. Observations on mammalian cells in vitro, confirmed by some data on plant material, suggest that the prenatal effects of nickel could partially be due to the production of certain changes in the mitotic apparatus provoking cellular death at critical times of development.