The transtubular potassium gradient (TTKG) is a simple physiologically based clinical test to study the renal excretion of potassium. This article reviews the most important physiological changes influencing TTKG, the hypokalaemia and hyperkalaemia, the effect of mineralocorticoids, alkalosis, action of diuretics among other drugs etc. The authors studied the abnormalities of TTKG occurring in clinical conditions (renal patients with nephrotic edema, "dry" patients with renal diseases, liver cirrhosis associated with ascites, and primary hyperaldosteronism) and compare them to the results obtained in healthy people. They consider the test to be useful in the recognition of conditions with hypoaldosteronism (including the various types of pseudohypoaldosteronisms and aldosterone resistance) and hyperaldosteronism as well as renal diseases, in accordance with the data published in the literature. On the basis of their own results, they found the method of determination of TTKG informative and helpful also when investigating the site of actions and the effect mechanisms of the diuretics.