Magnesium and calcium concentrations were measured by nuclear-absorption spectrometry (AAC-508, Hitachi, Japan) and selective ionometry (Microlyte, Kone, Finland) in the serum, red cells, lymphocytes, 24-h urine and exhaled air humor condensate in 23, 31 and 23 sibs (parents and their children) with preasthma, atopic bronchial asthma, free of these diseases, respectively. All the patients had magnesium deficiency and calcium hyperconcentrations in the blood cells, low urine excretion of magnesium. Parathormone and calcitonin serum concentrations underwent no changes. It is suggested that in contrast to changes of endobronchial homeostasis of bivalent cations, those of electrolyte cell composition are primary and appear prior to clinical manifestations of bronchial asthma.