1. The nocturnal polyuria syndrome (NPS) is characterized by an increased nocturnal urine output. The diurnal rhythm in the antidiuretic hormone (ADH) system is absent, and often there is no detectable ADH in the plasma at all during the night. The 24-hr urine output is normal or only moderately increased. Men without nocturnal micturition, normally have a substantial increase in their nocturnal plasma ADH, while those with a need to micturate during the night have the same ADH level at night as in the daytime. Women have lower ADH levels than men, and no nocturnal increase in ADH irrespective of nocturnal voiding. Subjects with an increased nocturnal voiding frequency due to increased nocturnal urine output have an increased thirst, most markedly at night. They often avoid drinking in the evening, but they are unable to resist the impulse to drink during the night. People with polyuria at night wake up often because of the need to void, and accordingly are often tired during the day. 2. An increased nocturnal urine output can be reduced by administration of desmopressin at night. In a short-term study of elderly sufferers from NPS, treated with 20 micrograms desmopressin as nose drops in the evening the nocturnal urine output was reduced from 65 +/- 8% of the 24-hr urine output before treatment to 50 +/- 15% during treatment. In another study elderly with NPS were treated with 40 micrograms desmopressin as an intranasal aerosol in the evening.(ABSTRACT TRUNCATED AT 250 WORDS)