In order to determine whether the metabolism of individual free fatty acids is affected by fasting, plasma levels were measured daily in seven obese females during ten days of fasting and four days of refeeding. There was a gradual rise in free fatty acids throughout the fasting period with some variability during the last three days. Free fatty acids remained high during early refeeding followed by a decrease at the end of refeeding. Changes in concentration were most pronounced for oleic (18:1w9) and palmitic (16:0) acids which had the highest initial levels. Expressed as percent change from baseline, oleic, palmitic, and linoleic (18:2w6) acids had similar patterns, while changes were more dramatic for palmitoleic acid (16:1) and less so for stearic acid (18:0) than the others. When expressed as proportion of total free fatty acids, there was very little change in any individual free fatty acid despite the large fluctuations in actual plasma values. It appears that the five major free fatty acids in plasma undergo similar changes during fasting and refeeding, and the palmitic and oleic acids can serve as suitable tracers for metabolic studies.