Ovarian blood volume was quantified by measuring optical density (414 nm) of haemoglobin in ovarian extracts and comparing it with that of known amounts of whole blood in the cyclic mature female rats during estrous cycle and periovulatory period. Haemoglobin in ovarian extracts had the same peak absorbance of 414 nm characteristic of oxyhaemoglobin in whole blood taken by cardiac puncture of the rats. There was a linear relationship between the absorbance and volume of whole blood in the sample. The ovarian blood volume was lowest on the day of metestrus and slightly increased on the day of diestrus. On the night of proestrus, the blood volume significantly increased and showed a gradual increase during preovulatory period from 0030 hrs to 0230 hrs and then started decreasing and reached a preovulatory level on the morning of estrus. Ovulation had occurred only in the rats sacrificed after 0230 hrs. Treatment of rats with indomethacin and propranolol significantly reduced the ovarian blood volume observed during the ovulatory period. Epinephrine and norepinephrine did not affect the ovarian blood volume. The results show that the ovarian blood volume changes significantly during the estrous cycle and reaches at maximum level at the time of ovulation which perhaps reflect vasodilatation and hyperemia associated with this process.