The anterior, medial and posterior hypothalamic nuclei in alloxan-diabetic male rats were studied by karyometry. Selective responsiveness of separate nuclei of the medial basal hypothalamus (MBH) and the anterior hypothalamus was revealed. In the MBH cell nuclear-size changes were most prominent in the ventromedial nucleus and less pronounced, but significant, in the arcuate nucleus. In the anterior hypothalamus a significant response was produced by the supraoptic nucleus. The authors do not exclude the possibility that the reaction of the neurosecretory cells of the supraotic nucleus is indirect and reflects disturbed vasopressin balance in alloxan-diabetic rats in response to impairment of water-salt metabolism. The responsiveness of the MBH nuclei is considered as evidence for their involvement, the bentromedial nucleus in particular, in hypothalamic control of the endocrine pancreas. Various means by which insulin-sensitive hypothalamic areas may be implicated in this control are discussed.