The reduction of vanadate (+5V) to vanadyl (+4V) was demonstrated by ESR spectra in the presence of methylene-blue, chlorpromazine, imipramine and dosulepin, but not in the presence of benzodiazepines and Li+. Bleomycine forms an (inactive) complex with +4V which may explain the disinhibition of the brain microsomal Na+-K+ ATPase in the presence of vanadyl. The reduction of +5V to +4V by antidepressants would diminish the binding of +5V to the ATPase and could account for some of the therapeutic action of the drugs in manic-depressive illness.