In order to verify if, in essential hypertension (EH), the microalbuminuria (AER) increase could be due to hemodynamic modifications or to glomerular structural changes, in 15 essential hypertensives (EHs) with 24-hour AER > 16 µg/min and in 15 EHs with 24-hour AER ≤ 16 µg/min, the day- and nighttime behavior of creatinine clearance (Ccr), as well as AER clearance (AER-C) and fractional clearance (AER-FC), and behavior of blood pressure (BP) was evaluated. Patients with 24-hour AER > 16 µg/min showed significantly higher values of 24-hour and daytime Ccr than the other group of EHs, while during the night period, there were no significant differences between the two groups. On the contrary, AER and both AER-C and AER-FC resulted markedly and significantly higher in the EHs with 24-hour AER > 16 µg/min not only in the 24-hour evaluation, but also during the nighttime study, notwithstanding the significant decrease in BP and in Ccr observed during the night. These data, in the absence of correlations between BP and AER-FC seem to demonstrate the existence in EHs with 24-hour AER > 16 µg/min of an altered glomerular permselectivity, due to changes of the glomerular me