Plasma citrate was determined in 12 obese subjects who underwent total fasting for 10 days. Mean plasma citrate concentration rose significantly from 128 before to 205 micro mol/1 on the 10th day of fasting. Plasma citrate rose continuously during fasting in seven subjects in whom daily determinations were carried out. The 24-hour urinary citrate excretion was followed in six subjects. A significant decrease was found from 2.91 mmol/24 h in the prefasting state to 0.25 mmol/24 h at the end of the fast. Intravenous glucose tolerance test were performed before and on the 10th day of fasting. Kivgtt decreased significantly and was inversely related to plasma citrate concentration on the 10th day of fasting. The results agree well with the concept that an increased citrate level of tissues is of regulatory importance for the decreased glucose utilization during fasting in man.