Twenty-three chronic undifferentiated schizophrenics, 13 women and 10 men, aged 37-64 years with 15-to 40-year histories of the disease were given either thyrotropin-releasing hormone (TRH) (10 subjects) or DDAVP (13 subjects) with the aim to improve the negative symptoms of the disease and memory. TRH (600 µg i.v.) and DDAVP (4 µg i.m.) were administered every other day for 30 days. Negative symptoms were monitored by the Andreasen rating scale and by the Honingfeld NOSIE rating scale, memory by the Folstein ‘Mini mental State’ rating scale and by the Luria-Nebraska rating scale before therapy and then at days 15, 16, 30 and 31 of treatment. Both therapies significantly improved negative symptoms. Memory was significantly improved in all the patients treated with TRH and in 9 of the 13 patients treated with DDAVP, who presented less severe cognitive impairments. A peripheral mechanism of action of DDAVP was excluded by the observation that plasma electrolytes and osmolality, blood pressure, ECG patterns, 24-hour urine volume and specific gravity, basal plasma cortisol and growth hormone levels and weight of the patients were unchanged during therapy. TRH treatment induced a transient borderline hyperthyroidism at day 15 and a progressive decrease of the thyrotropin response to TRH stimulation. A common mechanism of action of the two peptides on the central noradrenergic system is sugg