Hypomagnesaemia in a woman treated with theophylline and albuterol because of recurrent asthmatic attacks prompted us to explore the effects of these drugs on the metabolism of magnesium, calcium, sodium and phosphate in such patients. Theophylline given intravenously to 10 females with recurrent asthmatic attacks increased total mean urinary excretion (mean +/- SEM, mmol 5 h-1) of Mg from 0.54 +/- 0.07 to 0.86 +/- 0.10; of Ca from 0.89 +/- 0.18 to 1.45 +/- 0.26; of Na from 22.9 +/- 7.5 to 49.4 +/- 9.5. Theophylline i.v. and an inhaled beta 2-agonist (albuterol) both increased the normal morning-till-noon serum concentration difference (mean +/- SEM, mmol l-1) in PO4 (from -0.13 +/- 0.04 to - 0.23 +/- 0.03 and -0.23 +/- 0.04, respectively) and reduced the normal increase in serum-K (from 0.25 +/- 0.07 to 0.06 +/- 0.08 and -0.13 +/- 0.09, respectively). Disproportional changes in serum and urinary levels of magnesium and calcium by theophylline i.v. is suggestive of Mg depletion of intracellular stores and a negative calcium balance. Theophylline, therefore, may exert adverse effects on the metabolism and urinary excretion of calcium and magnesium in subjects with recurrent asthmatic attacks.