The mechanism of action of inhalational anesthetics is unknown, but neuronal membrane alteration is a favored hypothesis. Since phospholipid methylation and translocation play a key role in the transmission of biologic signals across cell membranes, we examined the effect of two commonly used halogenated anesthetics, halothane and isoflurane, on phospholipid methylation in rat brain synaptosomes. Using S-adenosyi-L-[3H-methyl]methionine as a donor, we found a two-fold increase in3H-methyl incorporation into phospholipids in synaptosomes taken from rats exposed to concentrations that just abolish pain response, but not in rats exposed to higher or lower concentrations. Methylation was not increased in rats newly recovered from anesthesia. Halothane added to synaptosomes taken from rats not previously exposed to anesthetics stimulated3H-methyl incorporation over a wide range of concentrations. Enhancement of phospholipid methylation by halothane and isoflurane may effect an alteration of neural signal transduction that results in the anesthetic state.