Cytochromes P450 play a key role in drug metabolism in man. Electrons are supplied to the cytochromes by NADPH through the flavoprotein NADPH-cytochrome P450 reductase (CPR). Electron transfer from CPR to cytochrome P450 is only possible in a membrane environment, but electron transfer to cytochrome c occurs at a comparable rate in solution, facilitating structural studies. We have identified mutants of CPR which bind cytochrome c more tightly than the wild-type, and have shown that, for at least some of these mutants, the CPR-cytochrome complex is sufficiently long-lived to be studied by solution scattering. We therefore plan to use SANS, with 2H-CPR and contrast-matching experiments, to study the cytochrome c complexes of CPR and of four tightly-binding mutants. This will allow us to obtain, for the first time, a structural model of the CPR electron transfer complex.