The renal selectivity properties towards albumin were evaluated in ten diabetic patients with arterial hypertension before and after the pharmacological normalisation of blood pressure, and were compared to 12 subjects with essential hypertension. While all patients of the control group were normoalbuminuric during hypertension, six of the diabetic group were microalbuminuric when hypertensive and became almost normoalbuminuric after blood pressure pharmacological control. All microalbuminuric diabetic patients presented altered properties of renal selectivity as epitomised by a non-preferential urinary excretion of glycosyl albumin (GA) (urinary GA/serum GA less than or equal to 1). At variance the selectivity properties were normal in normoalbuminuric diabetic patients and in essential hypertension. It was concluded that in diabetes mellitus arterial hypertension is associated with microalbuminuria when the renal properties of selectivity are altered, but does not implicate any proteinuric effect in those cases where the GBM function is preserved.