Single subcutaneous injections of each of five substituted hydrazines were given alone and jointly with pyridoxine hydrochloride (PH) to Swiss mice. The convulsive, toxic, and lethal effects of four compounds--methylhydrazine (MH), ethylhydrazine hydrochloride (EH), n-butylhydrazine hydrochloride (BH), and beta-N-[gamma-L(+)-glutamyl]-4-hydroxymethylphenylhydrazine--were successfully prevented by administering PH before and/or after injection. The toxic symptoms caused by N-methyl-N-formylhydrazine (MFH) were, however, only slightly inhibited by this vitamin.