Abstract Transfer of CrCl 3 to embryonic mice and changes in the embryonic mouse neuroepithelium associated with Cr-induced neural tube defects were studied. A single intraperitoneal injection of 51CrCl 3 was administered on the 8th day of gestation and animals were killed at various intervals for measurements of radioactivity in the maternal blood and embryos. Radioactivity per gram of fetal tissues increased during the observed period, while those of maternal blood decreased. Pregnant mice were injected intraperitoneally with a single dose of CrCl 3 (non-radioactive) on the 8th day of gestation. Animals were killed at specified times after injection and embryos were removed and examined histologically. At 4, 8 and 12 h after injection many pyknotic cells were observed on the neural plate, especially at 8 h after injection. These data suggest that CrCl 3 may affect embryos directly and cause neural tube defects and that pyknotic cells on the neural plate may be an early pathological finding in cases of developing exencephaly.