Drugs that induce an increased urine flow are used both legitimately (treatment of hypertension and oedema) and otherwise (rapid weight loss) in sports and exercise. There are 5 major categories of diuretic drugs based on their mechanisms and loci of action. Common to all classes is hypohydration, which has been shown to have an array of adverse effects on performance, including impaired strength, power and endurance. Postural hypotension can be particularly troublesome in the elderly. Also common to all diuretics, except those interfering with the aldosterone mechanism in the distal nephron, is hypokalaemia. Severe symptomatic hypokalaemia (serum K+ concentration less than 3.0 mmol/L) is rare except in clinical situations in which additional hypokalaemic factors are present. Moderate levels of hypokalaemia (serum K+ concentration 3.0 to 3.5 mmol/L) can increase the risk of adverse reactions as has been shown in a variety of prospective clinical studies. Hypokalaemia has effects on cardiac rhythm, muscle function and integrity, local blood flow, carbohydrate metabolism, and the blood lipid profile. Performance studies generally show diminished exercise tolerance in direct proportion to the degree of hypohydration induced. This is not the case, however, in a clinical setting of compromised cardiopulmonary function, in which diuresis has direct and indirect inotropic effects which augment exercise tolerance and decrease symptoms. The ability of the carbonic anhydrase inhibitor, acetazolamide, to induce a hyperventilatory response to the obligatory metabolic acidosis is taken advantage of in mountaineering to prevent or ameliorate the symptoms of acute mountain sickness, thereby improving exercise performance at high altitude. It is suggested that in clinical situations in which the use of a diuretic is considered appropriate, every effort be made to maintain or restore the serum concentration and the total body store of potassium to normal. To some degree this can be accomplished through diet, although potassium chloride supplements or potassium-sparing diuretics or diuretic combinations may be necessary.