I first became interested in biochemistry as a medical student in Cambridge and this led me to take the Part I1 biochemistry course in 1946-47. I would have been content to be either a biochemist or a physician, but at the critical points in my undergraduate and postgraduate education at Cambridge and at University College Hospital I was encouraged to attempt to be both. The view at the time was that medical research was in need of physicians who were professional scientists. There are comparatively rare individuals who can direct laboratory research at a challenging level from the bedside or who can divide their time between the two activities and suceed at both. 1 am not one of them. It is necessary for me to do experiments with my own hands in order to have ideas and to advise students and fellows in their work. Because of this I eventually gave up clinical work on appointment as Professor of Biochemistry at Bristol. My present appointment as a Clinical Professor in the Oxford Medical School is perhaps a recognition of the need for scientists in clinical schools-a healthy trend which may be lost if current deprivation continues. The Biochemical Society's award of the CIBA Medal is an honour which I value and appreciate deeply. My current research is concerned with the mechanisms which regulate reversible phosphorylation in the mitochondrial pyruvate dehydrogenase (PDH) and branched chain 2-oxoacid dehydrogenase complexes in relation to effects of diabetes and of diet. This lecture is concerned with the PDH complex. its role in fuel selection, and the mechanisms which may mediate effects of diabetes and diet on its activity.