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Angiotensin-converting enzyme inhibitors (ACEI) block degradation of bradykinin, and bradykinin stimulates prostacyclin synthesis. Therefore, we set out to determine whether the effects of ACE inhibitors on prostaglandin production in essential hypertensive patients are class effects or are dependent on ACE inhibitor structure. In addition, we studied whether hypertensives show an impaired capacity to synthesize vasodilator prostaglandins. To address these questions, we compared the effects of captopril (sulfhydryl-containing inhibitor), enalapril and ramipril (carboxyl-containing inhibitors) and fosinopril (phosphoryl-containing inhibitor) on blood pressure and urinary excretion of 6-keto-prostaglandin (PG) F1-α (the breakdown product of prostacyclin) in 44 mild-to-moderate essential hypertensive subjects before and 8 weeks after administration of an ACEI. We also studied prostacyclin excretion in 15 normotensive healthy controls. Levels of urinary 6-keto-PGF1-α (pg/ml) were measured by specific radioimmunoassay. Hypertensive subjects showed a lower excretion of 6-keto-PGF1-α than normotensive controls (212 ± 147 vs 353 ± 98 pg/ml, p < 0.001). All ACEI induced a significant decrease in MAP and increased the rate of excretion of the prostacyclin metabolite: C, 211 ± 200 to 338 ± 250 pg/ml, p < 0.05; E, 202 ± 133 to 296 ± 207 pg/ml, p < 0.05; R, 205 ± 127 to 342 ± 211 pg/ml, p < 0.05; F, 235 ± 128 to 347 ± 241 pg/ml, p < 0.05. In hypertensives (n = 44) the decrease in blood pressure correlated negatively with the rise in 6-keto-PGF1-α excretion (r = -0.51, p < 0.001). These data suggest that impaired prostacyclin biosynthesis in hypertensive patients could account for haemodynamic changes leading to the hypertensive state. Moreover, the hypotensive mechanisms of ACEI may be mediated by an increase in prostacyclin production; this effect seems to be class-dependent. [ABSTRACT FROM AUTHOR]