Weanling rats were fed, for 50 days, diets containing starch, sucrose, maltose, fructose or glucose. When compared with the effects of the high-starch diet, diets containing the sugars promoted a substantial, and similar, rise in plasma lipids. Hepatic lipogenesis was increased by all the sugars, sucrose and maltose having an equal effect. Lipogenesis in adipose tissue was, however, depressed by feeding sucrose or fructose. The sugars were digested and absorbed much more rapidly than was starch. The hyperlipidaemic property of sucrose is discussed in relation to these findings.