The cytochrome P450 enzyme system is a multicomponent electron-transfer chain composed of a haem-containing monooxygenase cytochrome P450 (CYP) and one or more redox partners. Eukaryotic CYPs and their redox partner NADPH-dependent cytochrome P450 oxidoreductase (CPR) are involved in many biological processes. Each protein has one N- terminal membrane anchor domain for location within the endoplasmic reticulum (ER). In mammals, CYPs and CPR are especially abundant in liver cells, where they play important roles in the metabolism of steroids, fatty acids, and xenobiotic compounds including numerous drugs of pharmaceutical importance. Incorporation into lipid membranes is an important aspect of CYP and CPR function, influencing their kinetic properties and interactions. In this thesis, soluble nanometer-scale phospholipid bilayer membrane discs, "nanodiscs", were used as a reconstitution system to study the influence of lipid membrane composition on the activities of the abundant human CYP3A4 and human CPR. Both enzymes were expressed and purified from bacteria, and assembled into functionally active membrane-bound complexes in nanodiscs. Nanodisc assembly was assessed by a combination of native and denaturing gel electrophoresis, and a fluorimetric assay was developed to study CYP3A4 reaction kinetics using 7-benzyloxyquinoline as substrate. Kinetic properties were investigated with respect to different lipid membrane compositions: phosphatidyl choline; a synthetic lipid mixture resembling the ER; and natural lipids extracted from liver microsomes. Full activity of the CYP3A4 system, with electron transfer from NADPH via CPR, could only be reconstituted when both CYP3A4 and CPR were membrane-bound within the same nanodiscs. No activity was observed when CPR and CYP3A4 were each incorporated seperately into naodiscs then mixed together, or when soluble forms of CPR were mixed with pre-assembled CYP3A4-nanodiscs. Thus, assembly of the two proteins within the same membrane was shown to be essential for the function of the CPR-CYP3A4 electron transfer system. Comparison of the reaction kinetics in different membrane compositions revealed liver microsomal lipid to have an enhancing effect both on the activity of the assembled CPR-CYP3A4 nanodisc complex, and on the activity of CPR alone incorporated in nanodiscs, when compared either to the synthetic lipid mixture or to phosphatidyl choline alone. Thus, natural lipids appear to possess properties or include components important for the catalytic function of the CYP system, which are absent from synthetic lipid. Input of electrons, measured by NADPH consumption, exceeded product formation rate by the CPR-CYP3A4 complex in nanodiscs, indicating "leakage" in the electron flow, possibly due to uncoupling of the two enzymes. Uncoupling was shown to occur by developing a novel fluorimetric method using the dye MitSOX to detect superoxide production. The significance of this, and to what extent control of coupling could be a natural means of regulation of the CPR-CYP system, remains to be determined. Thus, phospholipid bilayer nanodiscs prove a powerful tool to enable detailed analysis of the reaction kinetics of membrane-reconstituted CPR-CYP systems, and to allow pertinent questions to be addressed concerning the integral significance of the membrane environment.