These studies examine the regulation of plasma low density lipoprotein (LDL)-cholesterol levels by varying quantities of dietary saturated and polyunsaturated triacylglycerols. At a constant load of 0.12% cholesterol and 20% triacylglycerol, substitution of polyunsaturated for saturated triacylglycerols caused LDL receptor activity to increase from 25% to 80% of control and reduced the LDL-cholesterol production rate from nearly 200% to 155%. These changes caused the plasma LDL-cholesterol concentration to decrease from nearly 190 to 50 mg/dl. When the dietary content of each triacylglycerol alone was incrementally increased, the saturated lipid suppressed receptor activity while the polyunsaturated triacylglycerol increased receptor-dependent LDL transport. The magnitude of these effects was quantitatively similar, although oppositely directed. However, the saturated triacylglycerol also caused a dose-dependent increase in the LDL-cholesterol production rate and markedly increased the plasma LDL-cholesterol level while the polyunsaturated lipid did not affect either of these. These independent effects were also evident in experiments where it was found that substituting polyunsaturated triacylglycerol for saturated lipid increased receptor activity significantly more than did simply reducing the dietary content of saturated triacylglycerol. Thus, these studies show that triacylglycerols containing saturated or polyunsaturated fatty acids have effects on the major processes that regulate the plasma LDL-cholesterol level that are qualitatively and quantitatively distinct.