AbstractXenopus laevis embryos were exposed to hydrazine and its methylated derivatives, (mono)methylhydrazine, 1,2‐dimethylhydrazine (SDMH), and 1,1‐dimethylhydrazine (UDMH). Concentrations of hydrazine, methylhydrazine, and UDMH in excess of 10 mg/1 were teratogenic; SDMH was teratogenic at concentrations of 50 mg/1 or greater. Exposure to lower concentrations permitted survival and development into normal larvae. The period of embryogenesis during which exposure to hydrazine was teratogenic was during neurulation. Exposure to these compounds at later or earlier periods affected viability but was not teratogenic. The malformations affected the head, trunk, and tail. The most common abnormality observed was tail kinks. Other abnormalities were microcephaly, cyclopia, shortening of the trunk, and edema. All malformed embryos also had abnormal notochords. The possible relation of notochordal abnormality to the pathogenesis of other malformations is discus