In rats, of the Wistar-Porton strain, a single intravenous injection of 1.25 mg Cd2+ between days 9 and 15 of gestation results in a high incidence (80% of hydrocephalus, together with other malformations in the fetuses, examined on day 20. This dose is critical, since 1.1 mg Cd2+/kg is not teratogenic, while 1.35 mg Cd2+/kg kills all the embryos. Intravenous injection of Cd2+ to the pregnant rat on day 12 causes a dose-dependent inhibition of placental Zn2+ transport. At the teratogenic dose, Zn2+ transport is inhibited by about 75% at 4 hr. Thereafter, inhibition decreases with time but is still significant at 48 hr. At 20 hr after administration of Cd2+ the embryonic concentration of Zn2+ is depressed by 33%. In the whole embryo the activity of the Zn2+-dependent thymidine kinase is inhibited by about 60% at 4 hr and at 20 hr the DNA concentration is reduced significantly. Placental transport of 14C-leucine and 14C-uridine, as well as the embryonic incorporation of these precursors into protein and RNA is unaffected at least at short times after the administration of Cd2+. It is possible therefore, that the teratogenic effects of Cd2+ may be related to the inhibition of DNA synthesis in the embryo.