The objective of this paper was to find out if the higher night levels of vasopressin described in previous studies are a manifestation of a permanent and stable rhythm bound to the different periods of the day or if they are independent of them and due to other causes. Vasopressin secretion was studied in a group of seven healthy subjects with an inverted sleep--wake cycle (night workers who sleep and rest during the day). The study was performed during the last week of their working period after at least 3 weeks of continuous night shift. Plasma samples for vasopressin determination were taken every 4 h during a 24-h period while the subjects were performing their normal night work and with their usual sleeping habits during the day. Plasma osmolality, electrolytes and blood pressure were also assessed during the test. In contrast to previous studies where higher nocturnal values have been reported, we found significantly higher vasopressin levels during the day, giving as a whole a characteristic pattern with the highest vasopressin levels measured at 16.00 h followed by a progressive decrement that reached its nadir at 04.00 h. The total measured secretion of vasopressin was significantly higher during the day than during the night (p = 0.0313). No significant difference was found, on the other hand, between day samples, with the exception of samples taken at 16.00 h and at 12.00 h (p = 0.031). Plasma osmolality and electrolytes were within the normal range during the test and no statistical difference was observed at the various points. It was concluded that the secretion of vasopressin is higher during sleep and rest time and lower during the active part of the 24 h. The secretory pattern of vasopressin is not bound to the different periods of the day as such, nor to variations in plasma osmolality or electrolytes. It seems therefore reasonable to assume that the secretion of vasopressin has an intrinsic daily rhythm that is not related to known regulatory agents but is modulated by other unidentified factors. Several hypotheses are discussed.