The aim of this study was to investigate associations between plasma oxytocin and vasopressin concentrations and renal water and sodium excretion during normal pregnancy and gestational hypertension. Thirty-seven normotensive women and fifteen women with gestational hypertension were studied from week 12 to week 36. Hormones were analyzed with radioimmunoassay. Osmolality and sodium were analyzed by routine methods. Renal sodium excretion did not differ between groups or weeks and mean renal free water clearance was negative. Values for each group and hormone are presented in week 12 and week 36. In normotensive women, the vasopressin concentration was 1.1 ± 0.2 pmol/L and 0.7 ±0.1 pmol/L (P = 0.053), and in hypertensive women it was 1.7 ± 1.0 pmol/L and 0.7± 0.1 pmol/L (NS), resepectively. In normotensive women, the oxytocin concentration increased from 23 ± 1 pmol/L to 48 ± 3 pmol/L (P < 0.001). Corresponding values in hypertensive women were 36 ± 11 and 55 ± 5 pmol/L (NS). In normotensive women, the estradiol concentration was 441 ± 123 pmol/L and 4072 ± 338 pmol/L, while it was 192 ± 62 and 5536±836 pmol/L in hypertensive women. In normotensive women progesterone was 287 ± 158 nmol/L and 1699±369 nmol/L. In hypertensive women progesterone was 58 ± 8 nmol/L and 433±56 nmol/L, respectively. Conclusions, the low plasma vasopressin and increasing plasma oxytocin concentrations with unchanged water and sodium excretion indicate that oxytocin assists vasopressin in concentrating urine during pregnancy. The role of the lower plasma progesterone concentration in late pregnant hypertensive women is not clear.