To assess the influence of dietary fat composition on rates of oxidation of dietary myristic (MA) and palmitic (PA) acids, eight healthy males consumed prepared solid-food diets for 11 d with 40% of total energy as fat. Fifty-five percent of the energy obtained in the form of fat was provided as butter or beef tallow. On days 8 and 11 of each diet cycle, 20 mg/kg body wt of either [1-(13)C]MA or [1-(13)C]PA was ingested with breakfast. Hourly breath samples were collected over 9 h thereafter and 13CO2 enrichments were determined by using isotope-ratio mass spectrometry. The percentage of [13C]MA appearing in breath carbon dioxide over 9 h was more than twofold that of PA (P < 0.01). Diet fat composition did not influence the mean (+/- SEM) percentage 13C recovered over 9 h from either labeled MA (7.1 +/- 1.0% compared with 8.6 +/- 0.9% for butter and tallow, respectively) or PA (3.3 +/- 0.7% compared with 3.0 +/- 0.9% for butter and tallow, respectively). However, net MA oxidation, calculated as the percent recovery of fatty acids in the meal, was greater (P < 0.05) after the butter (329 +/- 45 mg) than after the tallow (212 +/- 25 mg) breakfast. In contrast, no difference was observed in net oxidation of dietary PA between butter (441 +/- 99 mg) and tallow (348 +/- 95 mg) meals. In conclusion, there was no effect of varying the dietary content of MA and PA on fractional oxidation; consequently, net oxidation of these fatty acids was proportional to their concentration within the diet.