The course of the hypothalamo-infundibular somatostatin containing pathway was visualized by immunocytochemistry (the unlabeled antibody peroxidase-antiperoxidase complex method) combined with various knife cuts in the hypothalamus. Somatostatin immunoreactive fibers left the perikarya in the anterior hypothalamic-preoptic periventricular cell mass in a lateral direction. After a loop-like path through the lateral hypothalamus, fibers enter the medial basal hypothalamus mainly in the lateral retrochiasmatic area near the ventral surface and project to the ipsilateral half of the median eminence and the pituitary stalk. Somatostatin immunoreactivity almost completely disappeared from the stalk-median eminence 3-7 days following a complete anterolateral transection while transections dorsal or posterior to the lateral retrochiasmatic area failed to influence it. 7 weeks after anterolateral deafferentation an increased density of somatostatin containing terminals and immunopositive perikarya were seen in the arcuate and ventromedial nuclei but these neurones failed to reinnervate the stalk-median eminence. Thus there appears to be a neuron system containing somatostatin-like immunoreactivity within the medial basal hypothalamus, cell bodies being most abundant in the posterior divisions of the ventromedial and arcuate nuclei. However, most if not all of the somatostatin immunoreactivity in the stalk median eminence originates from outside of the medial-basal hypothalamus. Further, we have demonstrated the importance of the careful histological evaluation of each individual operation if functional studies are to be carried out after hypothalamic deafferentation.