In an open triple crossover study in 8 patients with essential hypertension, the possibility has been investigated of whether the blood pressure lowering effect of hydrochlorothiazide 50 mg once daily was attenuated by co-administration for 4 weeks of ibuprofen 400 mg t.i.d., diclofenac 25 mg t.i.d. or sulindac 200 mg b.i.d. Only a slight, statistically nonsignificant change was found, with the exception of a significant increase in systolic blood pressure after 4 weeks treatment with ibuprofen. There was considerable variation in the blood pressure response during treatment with all three NSAIDs, with slight rises in blood pressure in 13 out of 24 periods. Body weight increased significantly on treatment both with ibuprofen and diclofenac, whereas the increase on sulindac was less and was transient. No significant change was found in various biochemical parameters, including plasma electrolytes, plasma renin activity (PRA), aldosterone, albumin and creatinine, in haematocrit or in the 24-h urinary excretion of sodium and potassium. The sole exception was a decrease in PRA during ibuprofen treatment. From these observations it is concluded that ibuprofen and diclofenac differ from sulindac in their interaction with the diuretic action of hydrochlorothiazide. It appears that all three NSAIDs can safely be combined with hydrochlorothiazide in hypertensive patients, but blood pressure should be monitored carefully when an NSAID are added.