The paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamic neurosecretory system have been extensively investigated by many workers. The functional aspects of vasopressin secretion (elaborated by the PVN and SON neurons) in relation to the vasculature of the anterior hypothalamus are also well documented. However, the available data concerning vasopressin (VP) functions are largely based on physiological studies. Corroborative morphological correlation with regard to this has received little attention. The present report elucidates the intricate anatomical relationships between the VP-neurons and the adjoining capillaries in the rat anterior hypothalamus. A peroxidase-antiperoxidase (PAP) immunocytochemical study, using a commercial VP antibody, was carried out for this purpose. The observations are interpreted from a functional standpoint. VP-immunostained elements, i.e. the somata and the processes (mainly dendrites), were localized (i) close to the wall, (ii) on the endothelium, and (iii) occasionally, in the lumen of the hypothalamic capillaries. The findings provide immunocytochemical evidence that the vasopressinergic elements are in direct relationship with the hypothalamic vasculature. This raises some interesting possibilities for the former to be involved in: (i) affecting the permeability of the blood-brain barrier for transport of various nutrient substances (important in aging and Alzheimer's disease), (ii) inducing an alteration in the water permeability of the brain vessels on which depends the precise adjustment of brain water content and of brain volume (fundamental to normal functioning of the brain), and (iii) serving as osmoreceptors of the blood flowing through the capillaries and thus providing a feedback mechanism for VP modulation.