Sixty patients were treated for 1 year for essential uncomplicated hypertension, 30 with beta-blockers alone (BB) and 30 with BB and chlorthalidone (CTD). BB did not affect serum K+ or Mg++. In the BB-group there was a statistically significant trend towards retention of Mg++ in a loading test, but the effect was clinically marginal. BB + CTD reduced serum K+ and Mg++ and caused significant Mg++ depletion, as shown by the Mg++ loading test. All the effects were highly significant and were clinically important. The metabolic perturbations due to CTD are potentially dangerous and make this drug unattractive as 'first choice' treatment for hypertension.