Triton WR 1339 has been used to block lipid removal from the serum of baboons. When Triton WR 1339 is given intravenously to baboons, there is a linear increase in the serum lipid concentrations over a 3-hour period. The rise in serum glyceride concentration is very marked, while cholesterol and phospholipid concentrations increase more slowly. The rates of secretion of glyceride, cholesterol and phospholipids into the serum have been calculated for baboons in the fasting state. The mean values for glyceride, cholesterol and phospholipid secretions in the male and female animals are not statistically different. In both male and female baboons, the rates of secretion of glyceride into the serum are about six times greater than the rates of secretion of cholesterol and phospholipid as determined in these experiments. Consumption of a diet containing 75% by weight of sucrose for a period of 4 weeks is associated with a rise in fasting serum glyceride concentration in male and female baboons. In those animals consuming 75% by weight of glucose, no significant changes occur in the fasting serum glyceride concentrations. Ingestion of a high sucrose diet produces no significant change in the fasting serum cholesterol and phospholipid concentrations, while in those baboons consuming the high glucose diet there is a significant fall in the serum concentrations of these lipid fractions. There is no measureable increase in the rate of secretion of glycerides, cholesterol or phospholipids into the serum in the fasting state after 4 weeks of consumption of high glucose or sucrose diets as compared with the control period.