To study myocardial norepinephrine (NE) activity in essential Hypertension 9HT) and to clarify its prognostic significance, we performed myocardial imaging with 123I-metaiodobenzylguanidine (123I-MIBG) and 201Tl at rest in 16 patients with HT and 9 normal subjects. In addition to ordinary tomograms, whole body images were obtained in both 123I-MIBG and 201Tl imaging. From the whole body image, a ratio of myocardial radionuclide accumulation to total injected dose was calculated (%Uptake). And Uptake Ratio (%Uptake of 123I-MIBG/%Uptake of 201Tl) was used as an index of myocardial 123I-MIBG uptake. Reduction of myocardial 123I-MIBG during 3 hours was calculated and expressed as washout rate (WOR). From the Bull's-eye map, the extent of defect was quantitatively assessed (Defect Score) and the homogeneity of the radionuclide distribution within the myocardium (CV) was calculated. In 123I-MIBG imaging, defect appeared in 12 patients with HT (75%), but in normal subject no one showed defect. Patients with HT were divided into two groups according to left ventricular ejection fraction (EF) responses to exercise stress which were obtained by 99mTc blood pool imaging. Group 1 consisted of 8 patients who showed EF elevation (5-10%) by exercise and group 2 consisted of 6 patients with depression of EF (-4-6%) by exercise. Rest EF, peak filling rate, exercise heart rate, exercise blood pressure and left ventricular mass were identical between two groups. Uptake Ratio in Group 2 (0.67 +/- 0.04) was significantly smaller (p < 0.05) than those in Group 1 (0.75 +/- 0.05) and normal subject (0.75 +/- 0.06). And in Group 2, WOR of 123I-MIBG was greater than that of Group 1. In Group 2, defect of 123I-MIBG appeared in all patients, but in Group 1 defect developed in 50%, besides Defect Score was significantly greater in Group 2 than in Group 1 (3.8 +/- 1.4 vs. 1.5 +/- 1.8, p < 0.05). The unhomogeneity of 123I-MIBG distribution was significantly greater in Group 2 (CV; 30 +/- 5%) than in Group 1 (21 +/- 4%) and in normal subjects (18 +/- 4%). These results suggested that quantitative analysis of 123I-MIBG imaging may be helpful for assessing the prognosis of HT.