Abdominal aortic constriction in rats results in mild cardiac hypertrophy (20% increase in left ventricular weight compared to sham-operated controls) which is associated with increased numbers of β-adrenergic receptors (123 ±7.3 ftnol/mg protein (mean ± SE) VS. 87 ± 4.3 ftnol/mg in controls,P< 0.01) without changes in their affinities for dihydroalprenolol. In vitro synthesis of phosphatidylcholine through successive methylation of phosphatidylethanolamine by S-adenosyl-L-methionine is enhanced in the hypertrophied myocardium (0.38 ± 0.03 nmol/mg per 30 minutes vs. 0.23 ± 0.03 nmol/mg per 30 minutes in controls,P< 0.01). In both experimental groups, methyltransferase activity has a high affinity for S-adenosyl-L-methionine (Km= 6.8 μM), depends on Mg2*, is optimal around pH 9.0, and is inhibited by S-adenosyl-L-homocysteine (Ki = 8.3 μM). The possible relationship between phospholipid methylation and changes in myocardial β-adrenergic receptors was studied in both normal and hypertrophied hearts. Preincubation of cardiac membranes with S-adenosyl-L-methi-onine increased the numbers of β-adrenergic receptors in proportion to the duration of incubation and the concentration of S-adenosyl-L-methionine. In both groups, S-adenosyl-homocysteine, but not 5'-AMP or L-methionine, attenuated the increase in β-adrenoreceptors. These results indicate that phos-pholipid methylation may be an important mechanism for regulation of β-adrenergic mechanisms in both normal and hypertrophied myocardium.Circ Res 47: 536-541, 1980