Selective venous blood sampling was performed in 89 patients with hypertension (14 pheochromocytoma, 10 Conn’s syndrome, 8 Cushing’s disease, 57 essential hypertension). We looked for diagnostic criteria and the valuability of blood sampling from the adrenal veins in such diseases. Defining a norepinephrine concentration of more than 8,000 ng/l as pathological, we had an accuracy of 94.6%. Defining an aldosterone concentration of more than 1,400 pg/ml as pathological, we had an accuracy of 97.4%. In Cushing’s disease this method was not very helpful due to overlapping results.