An earlier study (Biochim. Biophys. Acta 46 (1961) 205-216) failed to detect the enzymatic synthesis of phosphatidylcholine (PC) from phosphatidylethanolamine (PE) via a transmethylation pathway in rat small-intestinal microsomal membranes. This pathway was therefore assumed to be absent from this organ. Recently, however, in our laboratory it has been demonstrated that this pathway for the synthesis of phosphatidylcholine is present in rat colonic brush-border and basolateral membranes. It was therefore of interest to examine whether phospholipid methylation activity was present in rat small-intestinal brush-border membranes. The results of the present experiments demonstrate for the first time that this pathway for the synthesis of phosphatidylcholine exists in these plasma membranes. Evidence to support the enzymatic nature of this reaction include: loss of activity by heat denaturation and at 0 degree C, significant inhibition by S-adenosyl-L-homocysteine and saturation kinetics. The predominant product of this brush-border membrane phospholipid methyltransferase is phosphatidyl-N-monomethylethanolamine. This enzymatic activity has an apparent Km for S-adenosyl-L-methionine of 40 microM, a Vmax of 8.4 pmol/mg protein per 5 min, and a pH optimum of 8.0.