The study was designed to evaluate milk composition, content of synthesis precursors and milk fat constituents in response to the infusion of lipopolysaccharide (LPS) into external pudendal artery of lactating dairy cows. Six multiparous Holstein cows [(185±30) days of lactation, BW=（576±36） kg] were randomly divided into two group. A crossover trial design was used in the study, and cows in experimental group and control group were infused LPS (Escherichia coli O111∶B4, 0.01 μg/kg BW) and physiological saline into external pudendal artery, respectively. The study consisted of 2 experimental periods with 7 d each and a 14-day-intermission between. The results showed as follows: dry matter intake (DMI) was not significantly affected by LPS challenge (P>0.01); milk protein percentage was significantly increased (P>0.05), but milk fat percentage was not significantly affected by LPS challenge (P>0.05); LPS challenge also significantly reduced the plasma non-esterified fatty acid (NEFA) content (P<0.01), which was the synthesis precursor of milk fat, meanwhile, the hydroxybutyric acid (BHA) content tended to decrease at first and then increase (P>0.05); by the challenge of LPS, the contents of saturated fatty acids (P>0.05) and short-chain fatty acids (P>0.05) were decreased, but the ones of unsaturated fatty acids and middle-chain fatty acids were increased (P>0.05), in addition, the desaturation of fatty acids was also affected. It can be concluded that LPS is one of the major factors in affecting milk fat synthesis.［Chinese Journal of Animal Nutrition, 2011, 23（8）：1317 -1323］
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