In ten patients with hyperkalemic familial periodic paralysis, the changes in dynamometer strength, haematocrit and the plasma concentrations of electrolytes, protein, glucose, immunoreactive insulin, glucagon and catecholamines were followed during attacks induced by KCl-ingestion or exercise. Before the attacks, the plasma electrolytes, glucose and hormones were not significantly different from those measured in six control subjects. In the control subjects, the ingestion of KC1 (1.0-2.0 mmol/kg) led to a modest rise in plasma K, without weakness or changes in hormone concentrations. In all the patients, the ingestion of KCI (0.5-1.0 mmol/kg) induced a marked rise in plasma K, a similar decrease in Na, paralysis, and an increase in plasma protein and haematocrit. Whereas 1RI increased 2-fold, glucose, glucagon and catecholamines showed no consistent changes. During rest following exercise, plasma Na showed an early decrease followed by hyperkalemia associated with paralysis and haemoconcentration. The inhalation of salbutamol aerosol alleviated hyperkalemia and paralysis induced by exercise or KCl-ingestion, and treatment with salbutamol tablets prevented attacks. The paralytic attacks seem to be related to depolarization of the muscle cells with increased Na-influx. This is associated with an up to 20% reduction in plasma water. This abnormal response cannot be accounted for by deficient secretion of insulin or catecholamines. The beneficial effect of these agents on the attacks may be a result of stimulation of the active Na-K-transport and ensuing hyperpolarization of the cell membranes.