Oestrous rats were anaesthetized with pentobarbital, one of the femoral arteries and veins and one of the ovarian veins were cannulated, and a thin polyethylene cannula was fixed in the ovarian bursa. Five-min blood fractions were collected from the ovary for 50 min. Following the control fractions 15 mU of oxytocin, 15 mU of vasopressin or 50 microliter of 0.9% NaCl solution was given into the ovarian bursa over 10 min. Blood pressure and ovarian blood flow were continuously recorded. Progesterone (P) and oestradiol-17 beta (E2) were determined from the blood samples by RIA. Oxytocin did not alter the blood pressure, whereas the ovarian blood flow showed a short increasing tendency. Later, however, it started to decrease in parallel with the decrease in blood pressure owing to blood loss. The secretion of P and E2 remained unchanged. No changes in blood pressure were observed after vasopressin administration, although the ovarian blood flow quickly decreased in parallel with the secretion of P and E2. It is suggested that oxytocin has no direct effect on ovarian blood flow and hormone secretion in the rat. Vasopressin, however, is an effective vasoconstrictor in the rat ovary and may in this way reduce hormone synthesis.