RATS TREATED WITH ETHYNYLOESTRADIOL HAVE MARKED HYPOLIPIDAEMIA: serum cholesterol is decreased to 5%, triacylglycerol to 10% and phospholipid to 70% of control concentrations. Loss of serum cholesterol follows an exponential decay, with a half-life of 1.13+/-0.09 days. After 4 days of treatment, serum cholesterol concentrations remain relatively constant (ranging from 1 to 20mg/100ml) for at least 30 days. There is a concomitant 20-fold decrease in the d<1.21 fraction of serum proteins and a similar decrease in serum apolipoproteins as measured by sodium dodecyl sulphate/10%-polyacrylamide-gel electrophoresis. The activity of hepatic microsomal acyl-CoA-cholesterol O-acetyltransferase (EC 2.3.1.26) was significantly increased by ethynyloestradiol treatment (P<0.05). This activation caused hepatic cholesteryl esters containing mainly C(18:1) fatty acids to increase linearly as serum cholesterol concentrations decreased (r=0.9675, P<0.001). Triton WR-1339, a non-ionic detergent that inhibits lipoprotein catabolism, was used to estimate hepatic lipid secretion by measuring the increment in serum lipids after its administration. At 15h after Triton WR-1339 administration, serum cholesterol concentrations were increased equally in both control and ethynyloestradiol-treated rats. In contrast, the increment of serum triacylglycerol of treated rats was 40% of that found in control rats, indicating that ethynyloestradiol inhibits hepatic triacylglycerol secretion. Triton WR-1339 inhibited the oestrogen activation of hepatic microsomal acyl-CoA-cholesterol O-acyltransferase and restored hepatic cholesteryl ester concentrations to normal values. These data suggest that ethynyloestradiol and its pharmacological ;antagonist' Triton WR-1339 alter hepatic triacylglycerol secretion via a mechanism associated with changes in hepatic cholesterol esterification.