The GH response to insulin hypoglycemia (insulin 0.1 IU/kg i.v.) was studied under basal conditions and during a course of haloperidol therapy in 19 chronic schizophrenics, 15 hebephrenics and four paranoids (ten men and nine women, age 16--53 years). Haloperidol was given for 30 days, at a daily dose of 6 mg i.m., and the GH response to insulin-induced hypoglycemia was tested twice, before, and 10, 20, 30 days following the initiation of the treatment. The psychopathological features were controlled daily by two psychiatrists and by the ward staff and by the use of a Wittenborn rating scale, rated at the same intervals as the hormonal assays. From the results obtained it appears that in schizophrenic patients, GH secretion and response to insulin stimulus are extremely variable and are unaffected by haloperidol treatment. On the basis of the results obtained, the neurotransmitter-neurohormone regulation of GH secretion in schizophrenics is discussed.