The embryotoxicity and teratogenicity of methylhydrazine, 1,1‐dimethylhydrazine, and 1,2‐dimethylhydrazine were investigated with pregnant Fischer‐344 rats. The compounds were administered ip on d 6–75 of pregnancy. A dose‐dependent reduction in maternal weight gains occurred for all three compounds. A dose‐related teratogenic effect did not occur for any of the three compounds. Embryotoxicity, manifested as reduced 20‐d fetal weights, occurred only in the 1,1‐dimethylhydrazine and 7,2‐dimethylhydrazine high‐dose treatment groups. The results indicate that none of the three methylated hydrazine derivatives are selectively embryotoxic or teratogenic in the rat.