Elderly patients with hyperkalaemia often have low concentrations of plasma renin and aldosterone, perhaps secondary to reduced glomerular filtration and sympathetic insufficiency. The endocrine response to surgical stress and volume expansion during anaesthesia was studied in seven elderly patients with hyperkalaemia (mean age 87.7±SD 5.3 years), 18 elderly patients without hyperkalaemia (86.5±5.5 years), and 18 younger patients (52.6±7.2 years) as controls. Base-line values, in hyperkalaemic elderly patients, for plasma renin activity and plasma aldosterone concentration were 0.8±0.3 ng mL−1 h−1 and 2.8±0.8 pg mL−1 respectively (significantly lower than in the younger patients), and 287±42 pg mL−1 for plasma atrial natriuretic peptide levels, which were significantly higher. The plasma renin activity and aldosterone concentrations in elderly patients with hyperkalaemia were at all times lower, but not significantly, than those of the elderly patients without hyperkalaemia. The atrial natriuretic peptide concentrations (351±48 pg mL−1) in the hyperkalaemic elderly were significantly higher 90 min after induction of anaesthesia than in the normokalaemic elderly (108±38 pg mL−1). Hormone concentrations in the hyperkalaemic patients did not change during anaesthesia, but plasma atrial natriuretic peptide concentrations increased significantly in the normokalaemic elderly, and plasma renin activity and aldosterone of the younger patients increased significantly during anaesthesia. These results indicate that plasma renin activity, and the concentrations of aldosterone and of atrial natriuretic peptide in elderly patients with hyperkalaemia are unresponsive to surgical stress and volume expansion.