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 6-15 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 1,2-dimethylhydrazine high-dose treatment groups. The results indicate that none of the three methylated hydrazine derivatives are selectively embryotoxic or teratogenic in the rat.