1. The effects of two isoenergetic diets differing only in the values for polyunsaturated: saturated fat (P:S values of 0.2 v. 2.0) were studied in twenty adult human volunteers. 2. A period of 14 d on the high P:S diet failed to produce significant changes in fasting triglyceride levels, though there were individual variations. On the other hand, fasting cholesterol levels dropped by 10% (P less than 0.005). High-density-lipoprotein-cholesterol concentrations were not influenced by changes in the P:S value. 3. Investigations into the mechanism by which changes in the P:S value might affect plasma triglyceride values revealed no consistent effects on very-low-density-lipoprotein kinetics, insulin secretion, insulin sensitivity or free fatty acid concentrations. 4. The results of this study suggest that the largest increase in dietary P:S values that is likely to be obtained on a long-term basis may have only a small effect on plasma triglyceride and cholesterol concentrations.