Daily plasma citrate rhythms were followed during a 24-h day in seven subjects fed a common diet and from 0800 to 2100 hours in : (1) ten subjects fed a citrate-poor diet; (2) eight subjects who continued overnight fasting (short-term fasting) and (3) nine obese patients during the 7th day of a total fasting (long-term fasting). During feeding plasma citrate increased after meals (from 9.0 to 30.7%). Citrate profiles did not change in subjects receiving the common and the citrate-poor diet. Plasma citrate rhythms correlated closely to rhythms of plasma lactate but not to rhythms of plasma glucose, free fatty acids (FFA) or insulin in feeding state. During short-term fasting, plasma citrate increased and remained high during long-term fasting as did plasma FFA concentration. Individual plasma concentrations of citrate and FFA correlated positively in the postabsorptive state. During long-term fasting same relation became negative, while individual concentrations of citrate and glucose correlated positively. Daily urinary citrate excretion over three sampling periods did not change during feeding, decreased during short-term fasting and was constantly low on the 7th day of fasting. The study demonstrates endogenous daily plasma citrate rhythms during feeding which vary as do plasma concentrations of carbohydrate metabolites. During fasting, an increased plasma citrate concentration is proposed as a more direct measurement of metabolic adaptations underlying the decreased glucose utilization than is an enhanced plasma FFA level.