A modification of the Halász-Pupp neurosurgical knife was used in female rats to completely or partially isolate the medial basal hypothalamus from the adjacent central nervous. Changes in body weight and length, plasma GH levels, and GH metabolism were measured. Whereas complete or anterior knife cuts resulted in increased body length and weight, posterior cuts had no effect. At 7 months following surgery, non-stress plasma GH levels were not elevated in any group, including those exhibiting enhanced linear growth. Ether stress decreased mean plasma GH levels significantly in all groups except that with complete cuts; nevertheless, most animals with complete cuts responded to stress with decreases in plasma GH. GH turnover rates in heavy animals of normal length did not differ from those in intact controls. In contrast, animals that were both heavier and longer than controls exhibited a decreased turnover rate and increased t1/2 of 125I-labeled GH. Volumes of distribution of GH were comparable in intact and operated rats of normal length but markedly increased in those of exceptional length. Although no increase in non-stress plasma GH levels was observed at 7 months following surgery in animals