The increase susceptibility of the pregnant rat to intraperitoneally administered nickel (Ni) is apparent at 12 and 19 days of pregnancy and cannot be due, therefore, to the increase in total body weight. Teratogenic malformations occur when Ni is administered during organogenesis and are maximal at dose levels that are toxic for the dam. The yolk sac and chorioallantoic placentas accumulate Ni, but this does not prevent the transport of the metal to the embryo or foetus. The Ni concentrations in the conceptuses decrease more slowly with time than those in the maternal organs. In the foetuses, the decrease in concentration is due to the increase in weight, since the content of Ni increases between 4 h and 24 h. Foetal uptake of [14C]thymidine, [3H]leucine and 65Zn is unaffected at 3 h after the injection of the dam with 4 mg Ni/kg body wt. Incorporation of [3H]leucine into foetal protein, but not the incorporation of [14C]-thymidine into DNA, is decreased at this time. A major effect of treatment with this teratogenic dose is an increase in the maternal plasma glucose concentration which, in turn, alters the supply of the sugar to the foetus. The possible relevance of temporary foetal hyperglycaemia to teratogenesis is discussed.