New techniques have been developed employing microlight guides and miniature O2 electrodes which permit metabolic events to be studied noninvasively in periportal and pericentral zones of the liver lobule. These events include O2 uptake, fat and carbohydrate metabolism, monooxygen-ation and glucuronidation. The lobular distribution of maximal enzyme activities measured by immunohistochemical or microchemical techniques does not always correlate with metabolic flux rates as measured in periportal and pericentral regions with the new noninvasive methods. The region of the liver lobule exhibiting highest metabolic flux rates can be shifted to another region of the lobule by changing the direction of flow from anterograde to retrograde. These data are consistent with the hypothesis that many metabolic pathways (e.g., oxygen, carbohydrate and fat metabolism which are under dynamic short-term regulation) operate below maximal velocity in intact hepatocytes.