The relation between basal intrarenal hemodynamics and the renal response to acute inhibition of angiotensin-converting enzyme by captopril and albuminuria was assessed in 106 lean patients with essential hypertension without detectable proteinuria. It was observed that the microalbuminuric group (24.5% of the total population) was characterized by a higher systemic arterial pressure, a lower level of high-density lipoprotein cholesterol, and similar mean values of age, duration of hypertension, glomerular filtration rate, renal plasma flow, filtration fraction, and plasma renin activity when compared with normoalbuminuric subjects. In response to captopril, a significant renal vasodilatation without a change in glomerular filtration rate or a fall in filtration fraction was observed in normoalbuminuric patients only. In contrast, the renal vasodilator response was abolished in microalbuminuric subjects, together with blunting of the rise in plasma renin activity associated with captopril. This occurred despite similar indexes of activity of the endogenous renin-angiotensin system. It is suggested that microalbuminuria may be a marker of early functional or fixed intrarenal vascular dysfunction in never-treated lean patients with essential hypertension.