Plasma arginine vasopressin (PAV) concentration was determined by radioimmunoassay during the diurnal cycle in 8 recombent healthy male subjects. Two subjects were studied again 3 weeks later while receiving 1 mycles. In 8 out of 10 cycles, a nocturnal increase in PAV was found; there was a progressive rise during the night in 5 subjects and a peak occurred at 2400 or 3400 h. In 1 subject no variation was detected and in another, the pattern was compleetly different. The mean PAV in the 10 cycles was significantly (P less than 0.001) higher during the night than during the day. Dexamethasone did not modify the pattern of variation, but induced a significant (P less than 0.001) decrease in PAV. Hematocrit remained stable throughout the study as did osmolality, except at 2000 h, when a significant (P less than 0.001) increase (5 mOsm) on average occurred in every subject. Blood sugar, sodium or chloride did not account for the observed rise in osmolality and no simultaneous change in PAV occurred. A rise in PAV explains, to some extent, the known nocturnal decrease in urine output. Diurnal variations in PAV must be taken into account in clinical investigations involving vasopressin.