Characteristics of diuretics and approaches to preclinical investigations of substances with diuretic activity are considered. Experiments on animals must include assessment of the activity of new diuretics in comparison to standards and their influence on diuresis, glomerular filtration rate, and excretion of osmotically active solutes and particular ions. Approaches to evaluation of the locus of diuretic action in nephron using clearance methods are described, including the measurement of clearance and fractional excretion of various organic substances, ions, and solute-free water.