Various substances can interfere with the urinary activity of N-acetyl-beta-D-glycosaminidase (NAG), particularly certain bacteria that exert inhibitory or enhancing effects on the enzyme. Clearly, all this constitutes a limitation on the use of the technique. For these reasons, in patients suffering from interstitial and glomerular nephropathies and essential arterial hypertension a comparative study was carried out between the activity of urinary and serum NAG to establish if the latter produces more reliable results than the former. The results of urinary NAG activity confirm that there is no significant difference between healthy subjects and those with essential arterial hypertension; on the other hand, nephropathic patients present significantly higher urinary enzyme activity than healthy controls. On the other hand, evaluation of serum NAG does not seem to provide any practical aid as it does not highlight any difference between measurements carried out in healthy subjects and those carried out in hypertensive patients or those suffering from interstitial or glomerular nephropathy.