Activity of pyrimidine 5'nucleotidase was measured in hemolysate of erythrocytes of healthy persons and patients with essential hypertension. Cytidine 5'monophosphate (CMP) and uridine 5'monophosphate (UMP) were used as the substrates for evaluation of activity of so called I-isoenzyme and uridine 3'monophosphate (U3'MP) was used as a substrate for the II isoenzyme of Py5'Nd. It was found that the activity of Py5'Nd I was lower in hypertensives (26.8 mU/gHb (CMP)) and 69.3 mU/gHb (UMP) in comparison with normotensives (62.3 and 117.4 mU/gHb respectively) (p < 0.05). The activity of Py5'Nd II did not differ between studied groups. Possible metabolic consequences of decreased activity of Py5'Nd are discussed.