WHILE insulin can affect the function of the central nervous system (CNS) by producing hypoglycaemia, there is substantial evidence suggesting that insulin can also act directly on cells of the CNS to modify their function^1–4. In a study of the phylogeny of the insulin receptor, Posner⁵ reported specific binding of ¹²⁵I-insulin to membrane preparations from whole brain of rat, monkey and pigeon. Specific receptors for insulin have been detected in the hypothalamus of monkeys but not in the cerebral cortex or thalamus⁶; other regions of the CNS were not studied. We report here the presence of substantial concentrations of insulin receptors in discrete regions of the CNS of the rat. We detected specific binding of insulin in every area studied, although it differed by as much as five- to 10-fold among regions. The insulin receptor of the cerebral cortex was most extensively characterised, and by all criteria it was indistinguishable from the insulin receptor on classical target tissues (liver, muscle and fat) as well as other cells of humans, rodents^7–10, and other mammals and non-mammalian vertebrates.