A new isotope dilution LC-MS/MS method for assay of 7alpha-hydroxy-4-cholesten-3-one without need for derivatization is described. This method was used in catheterization experiments on healthy fasting volunteers. The levels of this generally used marker for bile acid synthesis were slightly but significantly higher in the hepatic vein than in the brachial artery. In contrast, the levels of the precursor to 7alpha-hydroxy-4 cholesten-3-one, 7alpha-hydroxycholesterol, were the same in the two vessels. It is concluded that there is a net extrahepatic metabolism of 7alpha-hydroxy-4-cholesten-3-one. The similarity and very high correlation between the levels in the two vessels (r=0.97) are consistent with the contention that 7alpha-hydroxy-4-cholesten-3-one is a suitable marker for the activity of the hepatic cholesterol 7alpha-hydroxylase and thus bile acid synthesis.