The role of clofibrate-inducible cytochrome P-450 IVA1 in the metabolism of endogenous lipids in both rat liver and kidney microsomal fractions has been investigated. 20(omega)-hydroxyarachidonic acid has been identified as a major metabolite after incubation with both tissue fractions and the structure confirmed by mass spectrometry. The arachidonic acid 20-hydroxylase activity is inducible by clofibrate in both liver and kidney, indicating that cytochrome P-450 IVA1 is probably the enzyme responsible for this activity. In addition, the kidney exhibited higher rates of arachidonate 20-hydroxylase activity than the liver (in both control and induced states). Although leukotriene B4 was also hydroxylated in the 20-position in both liver and kidney, clofibrate induction resulted in a decrease (approximately 50%) in hydroxylase activity. In addition, the absolute level of leukotriene B4 20-hydroxylase activity in both tissue homogenates and by purified cytochrome P-450 IVA1 in a reconstituted system, was 2-3 orders of magnitude lower than the corresponding activity for lauric acid and arachidonic acid as substrates, indicating that the leukotriene was not the preferred substrate for this enzyme. Computer modelling of the conformational geometries of the above three potential cytochrome P-450 IVA1 substrates have shown that both lauric and arachidonic acids adopt a compact, 'hairpin' structure that are almost superimposed on each other, thereby rationalizing why they are relatively good substrates for this isoenzyme. By contrast, leukotriene B4 adopts a more bulky geometry than the two fatty acids, thereby providing a coherent structural reason why it is a poorer substrate for the cytochrome P-450 IVA1 isoenzyme.