This paper is partly a brief academic autobiography. It begins in 1942 when I volunteered for lifesaving research for the Royal Navy. This brought me to a Department headed by a very unusual Professor, R.A. McCance, an eccentric and a polymath. I have tried to say something about him and the Department. After the war, McCance gave me the Ph.D. project: The Effect of Ventromedial Lesions in the Hypothalamus in Rats in Parabiosis. It had recently been discovered that such lesions cause obesity, and energy balance was an active field. Parabiosis dates to the nineteenth century but had not previously been used in this context. The results were uniform and dramatic. I have briefly presented them, with a review of my own and others’ subsequent work. This leads to a picture of a negative feedback system, which regulates food intake to maintain a near-constant proportion of fat in the body, and maintains energy balance with increasing precision as time progresses. The parabiotic effect strongly suggests that there must be a blood-borne link between body fat and the hypothalamus. I have tried to make the case as strongly as I can for further work to identify this link, which has obvious scientific and clinical importance.