Ten chronic undifferentiated schizophrenics, 6 men and 4 women, aged 28-63, with 6- to 31-year histories of the disease were given DDAVP to observe the effects of this neuropeptide on the prevalent negative symptoms of their illness. Patients were maintained on neuroleptic therapy and first given a 20-day course of placebo followed by 20 days of DDAVP i.m., 4 micrograms Andreasen Scale for assessment of negative symptoms, the Brief Psychiatric Rating Scale, the NOSIE Rating Scale and the Luria-Nebraska Rating Scale were administered to monitor negative symptomatology, behavior and memory before the study began, after placebo and after DDAVP administration. Patients were also given a growth hormone-clonidine test and in addition plasma basal concentrations of 3-methoxy-4-hydroxyphenylglycol (MHPG), homovanillic acid, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) were measured at the same intervals. DDAVP therapy induced a significant improvement of negative symptomatology and a trend toward improvement of short- to medium-term memory. No changes in homovanillic acid, MHPG, 5-HIAA and DOPAC, nor of growth hormone response to clonidine stimulation were observed.