SUMMARYOsmoregulation of vasopressin secretion was studied in eight healthy women in the follicular and luteal phases of the menstrual cycle. Basal plasma osmolality in the luteal phase was significantly lower than in the follicular period (282·4±0·6, 285·6 ± 1·1 mmol/kg, respectively,P<0·05). Plasma AVP (pAVP) and plasma osmolality (pOsm) were measured during the infusion of 850 mmol/1 saline in both phases of the cycle, and linear regression analyses of these data gave the following regression equations (i) follicular, pAVP = 0·43 (pOsm‐284),r=+0·93, (ii) luteal, pAVP = 0·31 (pOsm‐279),r=+0·95. Both the slope and abscissal intercept were significantly different (P<0·01). Osmotic threshold for thirst sensation in the luteal phase was lower than the follicular (293±2, 297±1 mmol/kg,P<0·005). We conclude that, in the luteal phase, the threshold for AVP release and the gain or sensitivity of the osmostat are reduced together with lowering of the thirst threshold, which account for the lower basal luteal