Although it is known that the fatty acid profile of human milk is altered by diet, the rapidity with which this occurs has not been addressed. We hypothesized that after absorption the fatty acids of a given meal would be transferred rapidly from the chylomicrons of the blood into human milk. Fourteen lactating women drank six test formulas, each containing a different fat: menhaden oil, herring oil, safflower oil, canola oil, coconut oil, or cocoa butter. The subjects collected a midfeeding milk sample before consuming the breakfast test formula and additional samples at 6, 10, 14, and 24 h and then once daily for 4-7 d. Fatty acids of special interest included eicosapentaenoic and docosahexaenoic acids from menhaden oil, cetoleic acid from herring oil, linoleic acid from safflower oil, linolenic acid from canola oil, lauric acid from coconut oil, and palmitic and stearic acids from cocoa butter. Each of these fatty acids increased significantly in human milk within 6 h of consumption of the test formulas (P < 0.001). Maximum increases occurred 10 h after safflower oil; 14 h after cocoa utter, coconut oil, canola oil, and menhaden oil (eicosapentaenoic acid); and 24 h after herring oil and menhaden oil (docosahexaenoic acid). All of these fatty acids remained significantly elevated in milk (P < 0.05) for 10-24 h, except for docosahexaenoic acid, which remained significantly elevated for 2 d, and eicosapentaenoic acid, which remained elevated for 3 d. These data support the hypothesis that there is a rapid transfer of dietary fatty acids from chylomicrons into human milk.