1. Two female and two male patients with essential hypertension were given dihydralazine plus saline or dihydralazine plus diclofenac intravenously on two separate occasions. The dihydralazine dose (range: 16.2–32.4 mg/2 h) was chosen individually to reduce systolic blood pressure by 20 mmHg from the control value before the investigation; the intravenous diclofenac dose was 75 mg/20 ml of saline for all patients.

2. Dihydralazine decreased diastolic blood pressure more than systolic blood pressure and increased heart rate; diclofenac reduced both of these effects.

3. Dihydralazine increased urinary volume and sodium clearance; both these effects were antagonized by concomitant treatment with diclofenac.

4. Whereas dihydralazine did not influence creatinine clearance appreciably, concomitant administration of diclofenac reduced creatinine clearance.

5. Urinary prostaglandin E2 was reduced by diclofenac.

6. We suggest that dihydralazine-mediated vasodilatation is reduced by cyclo-oxygenase inhibition.

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