ABSTRACT We studied the changes in blood plasma electrolytes during acute and short‐term treatment with four different β‐adrenergic‐blocking agents in 31 patients with hyperthyroidism. Serum phosphate increased during the first four hours and remained elevated after one week both after cardioselective (acebutolol) and non‐cardioselective (oxprenolol, pindolol and timolol) β‐blockade (p<0.05). Albumin decreased after four hours but no change was found after one week. Serum creatinine increased after one week in the acebutolol‐treated group, while sodium, potassium and albumin‐corrected calcium in serum did not change. The hyper‐phosphatemic effect may be due to blockade of β₁‐adrenoceptors. since the effect of the cardioselective acebutolol was no less than that of the non‐cardioselective β‐adrenoceptor blockers. 1987 Association for the Publication of the Journal of Internal Medicine