Factors reported to influence the plasma triglyceride concentration include the amounts and type of carbohydrate and fat and the caloric intake. The effects must be exerted through changes in triglyceride formation, removal or both, which in turn may be influenced by changes in free fatty acid (FFA) turnover, insulin secretion or body weight. These factors have been studied under the following circumstances: (1) comparison of diets rich in either sucrose, starch, glucose or fructose; (2) comparison of diets rich in either saturated or polyunsaturated fatty acids; and (3) comparison of eucaloric, hypercaloric and hypocaloric diets. The triglyceride concentration, the insulin responsiveness to glucose, and the total, though not the fractional rate of appearance of radioactive FFA in triglyceride fatty acids (TGFA) were always greater with sucrose than with starch; the formation of TGFA therefore appeared to be greater with sucrose though the higher triglyceride levels showed that removal was also less efficient. Glucose and fructose raised triglycerides to a similar extent but the rate of appearance of FFA in TGFA seemed to be greater with glucose. Saturated fats invariably led to higher triglyceride levels although the rate of incorporation of FFA into TGFA was one the average less than with polyunsaturated fats, suggesting that saturated fats gave rise to a relative diminution in the removal of triglycerides. Studies carried out sequentially during carbohydrate-rich diets showed that the rise in triglyceride levels was associated at first with an increase and later with a decrease in the incorporation of plasma FFA into TGFA. With starvation there was a fall in the triglyceride concentration which was however associated with an increase in the total rate of appearance of FFA in TGFA. Plasma FFA turnover was also raised but insulin responsiveness to glucose fell. Overeating led to a marked rise in triglyceride concentration and the insulin response; the total rate of appearance of FFA in TGFA increased only modestly whereas the fractional rate of equilibration of FFA with TGFA fell strikingly, suggesting that inadequate removal was a major cause of hypertriglyceridemia. Among the subjects studied on eucaloric carbohydrate-rich diets the plasma triglyceride concentration was significantly related to the rate of appearance of FFA in TGFA, to FFA turnover, to the insulin response and to the ponderal index. The insulin response and the ponderal index were also significantly correlated.