We studied the neurohypophyseal function of 20 patients with complete GH deficiency owing to pituitary stalk transection by means of the water deprivation and the hypertonic saline infusion test. In T1-weighted magnetic resonance images, high-signal intensity of the posterior lobe of the hypophysis was missing in all the patients. An ectopic posterior lobe was observed at the proximal stump of the transected stalk in 17 patients, whereas the 3 patients without ectopic posterior lobe had overt diabetes insipidus or intractable nocturnal enuresis. After water deprivation, 3 patients with large ectopic posterior lobes (length along the pituitary stalk axis exceeding 5 mm) showed urinary osmolality and plasma AVP levels as high as those of 13 patients with partial GH deficiency in whom magnetic resonance images revealed no abnormalities. On the other hand, 14 patients with small ectopic posterior lobes (length less than 5 mm) showed significantly lower urinary osmolality and plasma AVP levels than the patients with large ectopic posterior lobes (p less than 0.01 and p less than 0.01, respectively) and the patients with partial GH deficiency (p less than 0.01 and p less than 0.01, respectively). Urinary osmolality in the patients with small ectopic posterior lobes, however, was higher than that in 3 patients without ectopic posterior lobes (p less than 0.01). During the hypertonic saline infusion test, peak plasma AVP levels in the patients with small ectopic posterior lobes were significantly lower than those in the patients with partial GH deficiency (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)