The injection of a large bolus of angiotensin II causes a biphasic blood pressure response in the conscious rabbit. To investigate contribution of prostaglandins (PGs) to the depressor phase of the blood pressure response, we studied the blood pressure effect of i.v. bolus injections of angiotensin II before and after the administration of an inhibitor of cyclooxygenase, indomethacin or sodium meclofenamate (10 mg kg-1), and in relation to associated changes in the plasma concentration of immunoreactive PGs. In conscious rabbits, angiotensin II (0.05-5.00 microgram kg-1) produced a dose-related pressor response which at both 1.5 and 5.0 micrograms kg-1 was followed by lowering of blood pressure to below the preinjection level. Neither indomethacin nor meclofenamate affected the maximal rise in pressure produced by angiotensin II, but both cyclooxygenase inhibitors augmented the duration of the pressor phase and abolished the depressor phase of the hemodynamic response to angiotensin II at 1.5 to 5.0 micrograms kg-1. After administration of angiotensin II, 5 micrograms kg-1, the plasma concentration of 6-keto-PGF1 alpha increased (P less than .01) from 218 +/- 21 pg/ml by 221 and 235% during the pressor and the depressor phases of the blood pressure response, respectively. Increments in plasma 6-keto-PGF1 alpha correlated inversely with the duration of the pressor phase and directly with the maximal lowering of blood pressure during the depressor phase. Plasma levels of PGE2 and PGF2 alpha also were increased by the peptide but the increments were not correlated with any aspect of the blood pressure response. These data suggest that a mechanism involving PGs both curtails the pressor phase and mediates the depressor phase of the hemodynamic response to pharmacological doses of angiotensin II in the conscious rabbit.