It has been studied in dogs anaesthetized with chloralose-urethane whether Prostaglandin E2(PGE2) might act as a mediator of the stimulatory effect of ACTH on ovarian blood flow. Without any damage to the ovarian vessels, a heated thermocouple was inserted into the stroma of the ovaries and local blood flow was recorded continuously. Similarly to ACTH, PGE2 (100 ng/kg/min) infused into the ovarian bursa increased local blood flow. The ovaries were isolated in situ and perfused by a peristaltic pump from the femoral artery. Synthetic ACTH or PGE2 was infused into the ovarian artery for 15 min and perfusion pressure was measured. Both ACTH (12.5--25.0--50.0--100.0 ng/kg/min) and PGE2 (3912--6.25--12.50--25.00 ng/kg/min) reduced perfusion pressure, caused vasodilatation. There was a significant linear correlation between the degree of vasodilatation and the logarithm of the doses. If the doses of the two agents are expressed in terms of mols, the effect of both agents can be characterized by a common equation. Fifty--200 mug/kg indomethacin inhibited the effect of ACTH but increased the effect of PGE2. It is suggested that in dogs ACTH exerts its ovarian blood flow stimulatory effect by releasing PGE2 or some similar agent(s).