Diets rich in starch, sucrose or fat were fed to rats for various periods in order to investigate the relationship between plasma lipids and tissue lipogenesis. The high-fat diets supressed lipogenesis in the liver and adipose tissue, and reduced the plasma triglycerides (relative to values obtained in the starch-fed group). The high-sucrose diet greatly elevated hepatic lipogenesis without appreciably affecting the fat content of the liver, reduced fat synthesis in the adipose tissue, and raised the plasma triglycerides and (at 50 days) plasma cholesterol. These adaptive changes were evident throughout the lives of the animals. It is concluded that the hyperlipidaemia associated with sucrose feeding is a reflection of the increased throughput of triglycerides from the liver to the adipose tissue.