Phosphatidylethanolamine N-methyltransferase (PMT) in neuronal nuclei of rat brain was characterized. The neuronal nuclei (Nl) were isolated by discontinuous sucrose density gradient and the methylation of phospholipids was measured by the incorporation of [³H]-methyl group from S-adenosyl-L-[methyl-³H]-methionine (SAM). The characteristics of N1 PMT were compared to those of other PMTs. On the basis is exogenous phospholipid requirement, affinity for SAM, Mg^(2+)-dependence, and pH optimum, the properties of N1 PMT appears to be different not only from the enzymes of other rat tissues but also from those of the other subfractions of rat brain, mainly synaptosomes and myelin fractions.