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Conjugated linoleic acids (CLA) are potent anticarcinogens secreted into the milk of ruminants. The present study was carried out to detect the post–ruminal effects of a duodenally infused commercial CLA product on milk–fat synthesis and fatty acid pattern in lactating cows. Four Holstein cows, fistulated with duodenal cannulae, were infused with 200 g of a CLA mixture daily into the duodenum over a period of 5 days (period II), while no application of CLA acted as controls in period I and III. In contrast to the marginal effect of the CLA application on milk yield, an obvious decrease of milk fat content of 40% was measured. The fat yield decreased during the infusion period to about half of the control period, whereas protein and lactose content in milk was not significantly influenced. The CLA percentage of milk fat increased sevenfold during infusion of the CLA supplement. The data demonstrate that duodenal infusion of CLA inhibits the milk fat synthesis, resulting in the decrease of milk fat content and fat yield. It can be concluded, that CLA supplements increase the CLA concentration in milk fat, which probably has to be rumen protected for oral administration.