Levels of growth hormone (GH) are reduced in the genetically obese Zucker rat, fa/fa, in comparison to lean littermates. In normal rats, GH release is regulated by stimulatory and inhibitory factors of hypothalamic origin. The present experiment focuses on hypothalamic somatostatin (SOM; growth hormone release inhibiting factor) in order to determine if abnormal hypothalamic SOM may be a correlate of depressed GH secretion in fa/fa rats. We compared immunocytochemical localization of hypothalamic SOM between 5 obese (fa/fa) Zucker rats and 5 non-obese littermates. Brain sections from pairs of animals were processed simultaneously. The distribution of SOM immunoreactive cell bodies in the hypothalamus agreed with previous reports. SOM-containing neurons in the periventricular area were counted and analyzed at 4 hypothalamic levels: (1) anterior to the suprachiasmatic nucleus (SCN); (2) through SCN; (3) between SCN and the ventromedial hypothalamic nucleus (VMH); and (4) through VMH. The greatest number of SOM-immunoreactive cell bodies was observed at levels (2) and (3). The numbers of SOM-containing cells did not differ significantly between obese and lean animals. No apparent difference in density of fiber staining was observed in the median eminence.