The existence of the major biosynthetic and catabolic pathways of intermediary metabolism in the mammalian kidney has been known a long time. Differences between cortical and med-ullary metabolism have suggested further differences in the enzyme distribution along the course of the nephron. Such information was initially provided qualitatively by histochemistry. With the advent of microdissection and quantitative micro-chemistry [1] precise information concerning enzyme distribution along the course of the nephron has become available and also refined by determining the activity of those enzymes likely to be rate-limiting for the major metabolic pathways. Since an earlier review [2] new information has accumulated extending our knowledge to several more metabolic pathways, and to the sites of hormone action in different nephron segments. In addition, biochemical data can now be correlated with quantitative morphological data [3]. The technique of microdissection of the intact nephron [4] and the localization of hormone-specific adenyl cyclase [5] has led to studies that will eventually correlate enzyme activity, hormone action, and transport rates in individual nephron segments. Also, many biochemical functions described in the kidney do not seem directly related to reabsorptive or excretory functions. Drug metabolism, steroid hormone hydroxylations, renal gluconeogenesis as well as renal choline and triglyceride metabolism may be better understood by the knowledge of their precise localization along the nephron.

We have chosen to present enzyme activities in the form of a relative distribution pattern because this simplifies comparison of work from different laboratories and allows a correla-tion with morphological data. Thus, the segment with the great-est activity for that enzyme is designated as 100% and the proportion of this value is shown for each remaining segment. All enzyme activities were related to tubular dry weight or pro-tein. Literature data cited in relation to tubular length were converted to the present data using protein or dry weight values given in the literature and summarized elsewhere [61. For absolute activities and details of methods used, the reader is referred to original articles cited in the text and [61. Species differences have been considered where information is available.