Hypophysectomy in rats induces a decrease in VLDL and HDL and an increase in LDL. To investigate the importance of lecithin:cholesterol acyltransferase (LCAT) for these changes, plasma LCAT activity was determined, both according to the methods of Stokke-Norum and Glomset-Wright. Plasma LCAT activity as assessed by both these methods decreased significantly after hypophysectomy, indicating a reduction in plasma enzyme concentration. Plasma total and free cholesterol increased slightly, while the concentration of cholesteryl esters and triglycerides remained unchanged. Blood sugar and FFA decreased. The osmotic resistance of the red cells was increased, and target cells were seen. There was a decrease in blood haemoglobin and haematocrit. Serum total protein concentration, electrophoresis, and plasma ALAT activity did not change. The observed changes in lipoproteins and LCAT following hypophysectomy can be partly explained by decreased liver secretion of both LCAT and VLDL. Because of the complex relationship between lipoproteins and LCAT, no clear-cut conclusions about the association between decreased HDL and LCAT can be drawn. The precise hormonal mechanisms controlling LCAT activity and lipoprotein metabolism remain to be solved.