Dietary saturated fatty acids are implicated as a risk factor for atherosclerosis. The conversion of the major dietary saturated fatty acids stearic acid (18:0) and palmitic acid (16:0) to monounsaturated fatty acids in whole plasma and lipoprotein fractions is reported for seven healthy adult humans over 6 d using [U-13C]stearic acid (18:0*) and [U-13C]palmitic acid (16:0*) and high-precision mass spectrometry. A tracer dose (28-32 mg) of 18:0* or 16:0* was loaded into an emulsion and orally administered before breakfast. Serial blood samples were collected on day 1 and fasting blood was drawn daily until day 7. Overall conversion of 18:0 to 18:1 was approximately 14%, whereas that of 18:0 to 16:0 was approximately 2% in plasma up to 144 h. Conversion of 16:0 to 16:1 was < 2%, whereas conversion of 16:0 to 18:0 was approximately 6%. No other fatty acid metabolites were detected for 18:0* or 16:0*. The conversion products were observed mainly in chylomicrons and very-low-density lipoproteins, indicating that the intestine and liver have comparable roles in desaturating 18:0 and 16:0. Overall, these data indicate that dietary 18:0 desaturation is severalfold greater than 16:0 desaturation. The low level (14%) of 18:0 desaturation in omnivorous adults may have little influence on blood lipid profiles relevant to atherosclerosis risk.