In 20 patients with essential hypertension the urinary albumin execretion, glomerular filtration rate (GFR),and renal plasma flow (RPF) were examined before and after antihypertensive treatment. Albumin excretion measured by radioimmunoassay was increased before treatment, and there was a significant fall during treatment. In patients responding well to therapy (diastolic pressure below 100 mm Hg), albumin excretion was significantly lower than in patients responding poorly to therapy. There was a positive correlation between albumin excretion before treatment and diastolic pressure during treatment, indicating that the albumin excretion rate may be used to predict the result of antihypertensive treatment. Patients with excretion rates below 25 mug/min generally respond well to the treatment used. No definite changes in GFR and RPF were found during treatment, and there was no correlation between albumin excretion and GFR and RPF. It is suggested that the increased albumin excretion in essential hypertension is due both to functional and morphological alterations in the glomerulus, namely increased glomerular filtration pressure and vascular damage.