The activity of the soluble form of phosphatidic acid phosphatase in rat liver was stimulated about 2.5-fold by inclusion of mevinolin, a competitive hydroxymethylglutaryl-CoA reductase inhibitor, in the diet (0.1%). The stimulatory effect of mevinolin was present also after dietary addition of cholestyramine (5%) or intraperitoneal administration of ethanol. Addition of cholesterol (2%) to the diet totally abolished the stimulation by mevinolin on phosphatidic acid phosphatase. The results support a correlation between the synthesis of the rate-limiting enzyme in cholesterol biosynthesis and the activity of the apparent rate-limiting enzyme in triacylglycerol biosynthesis.