The effect of hyper- and hypothyroid, hypophysectomy and adrenalectomy on phosphatidylcholine biosynthetic enzymes, phosphatidylethanolamine methyltransferase, phosphatidyldimethylethanolamine methyltransferase and choline phosphotransferase of liver microsomes was measured in rats. There was a significant increase in the specific activity of phosphatidylethanolamine methyltransferase in the hyperthyroid rats. There was a significant reduction in the specific activity of phosphatidylethanolamine methyltransferase and phosphatidyldimethylethanolamine methyltransferase in the hypothyroid states. The choline phosphotransferase increased significantly in the hyperthyroid state and decreased in the hypothyroid animals. Hypophysectomy resulted in a significant increase in specific activity of choline phosphotransferase. A reduction in the specific activity of the phosphatidylethanolamine methyltransferase occurred after 28 days of hypophysectomy. Adrenalectomy resulted in a significant stimulation of the specific activity of phosphatidylethanolamine methyltransferase and choline phosphotransferase in liver microsomes.