Adult obese human subjects with a normal or slightly disturbed oral glucose tolerance test, were submitted to a 7-day fast. Initial serum triglyceride levels were inversely related to the Intralipid fractional removal rate, reflecting the close dependency of triglyceridemia on clearance efficiency; the correlation was less significant on the 5th day of fasting. The direction of the change in triglycerides during fasting was clearly related to the prefast triglyceride levels, the groups of patients with a lower level showing an increase, the group with a higher level showing a decrease. Changes in triglyceridemia were inversely related to fractional removal rate; changes in lipid clearing from plasma, however, could not explain the direction of changes in triglyceridemia in all subjects investigated. The group of patients responding to fasting with a decrease in triglyceridemia had a lower mean initial serum cholesterol value and their serum cholesterol levels showed a less prolonged increase during fasting than the other groups of patients.