Potassium is an essential electrolyte. Potassium balance is crucial for regulating the excitability of nerves and muscles and so critical for regulating contractility of cardiac muscle. Although the most important changes seen in the presence of deranged potassium are cardiac, smooth muscle is also affected with increasing muscle weakness, a feature of both hyperkalaemia and hypokalaemia. Physiologically, it exists as an ion in the body. Potassium (K+) is a positively charged electrolyte, cation, which is present throughout the body in both intracellular and extracellular fluids. The majority of body potassium, >90%, are intracellular. It moves freely from intracellular fluid (ICF) to extracellular fluid (ECF) and vice versa when adenosine triphosphate increases the permeability of the cell membrane. It is mainly replaced inside or outside the cells by another cation, sodium (Na+). The movement of potassium into or out of the cells is linked to certain body hormones and also to certain physiological states. Standard laboratory tests measure ECF potassium. Potassium enters the body rapidly during food ingestion. Insulin is produced when a meal is eaten; this causes the temporary movement of potassium from ECF to ICF. Over the ensuing hours, the kidneys excrete the ingested potassium and homeostasis is returned. In the critically ill patient, suffering from hyperkalaemia, this mechanism can be manipulated beneficially by administering high concentration (50%) intravenous glucose. Insulin can be added to the glucose, but glucose alone will stimulate insulin production and cause movement of potassium from ECF to ICF. The stimulation of alpha receptors causes increased movement of potassium from ICF to ECF. A noradrenaline infusion can elevate serum potassium levels. An adrenaline infusion, or elevated adrenaline levels, can lower serum potassium levels. Metabolic acidosis causes a rise in extracellular potassium levels. In this situation, excess of hydrogen ions (H+) are exchanged for intracellular potassium ions, probably as a result of the cellular response to a falling blood pH. Metabolic alkalosis causes the opposite effect, with potassium moving into the cells.