The left ventricular function of the heart was examined by means of the method of equilibrium-radionuclide ventriculography in 40 patients with essential hypertension (EH)--20 patients in stage I (H1), 20 patients in stage II (H2) according to WHO criteria--and in 18 normotensives (N). The examination was performed at rest and immediately after stress by intravenous infusion of hyperosmolar mannitol. At rest, the parameters of global systolic and diastolic functions of the left ventricle in normotensives do not differ significantly from the values in both groups of hypertensives. The global ejection fraction (GEF), peak ejection rate (PER) and peak filling rate (PFR) were in H2 significantly lower than in H1. Sectorial ejection fraction (SEF) in the apicoseptal area is in H2 significantly lower than H1 and N. After infusion of hyperosmolar mannitol the GEF and PFR increased in N and H1 only. When comparing all groups after infusion of mannitol the PFR in H2 is significantly lower also in comparison with N with the tendency (significant limit) to lower values of GEF and PER. PER and PFR were significantly lower and the end-diastolic volume (EDV) was significantly higher in H2 in comparison with H1. SEF was significantly lower in H2 in comparison with N in 3 out ot 9 sectors and in comparison with H1 in 8 out of 9 sectors. The infusion of hyperosmolar mannitol reveals subclinical disturbances of the diastolic and partially also of the systolic function of the left ventricle in stage II of EH which is very useful for diagnosis and treatment. (Tab. 4, Fig. 3, Ref. 22.).