Pregnant rats were housed in 85Kr atmospheres at 10, 15, or 20 days of gestation (dg) and killed after 4 hr of exposure to 37-40 nCi/ml. The 85Kr was present in the components of the fetoplacental unit (FPU) at concentrations (nCi/g) equivalent to approximately 2% of the concentration (nCi/ml) in the exposure atmosphere. Tissue distribution of 85Kr and the distribution of radiation dose did not suggest any unusual hazard to the fetus associated with exposure of pregnant animals. This conclusion was tested using 5-day exposures to a 1000-fold increased concentration: 40 muCi/ml. The main effects observed in pregnant rats exposed to 85Kr from 7-12 or 12-17 dg (estimated radiation dose of 5 X 10(3) rad to maternal lung and 5 X 10(5) rad to maternal skin surface) were deaths, impaired weight gain and skin lesions. Secondarily, the maternal toxicity led to indications of embryotoxicity, although the incidence of malformations was not increased by the estimated 50-rad dose to the FPU.