Changes in serum electrolytes and indices of renal function were studied in 187 patients with heart failure, aged 60 to 92 years, treated with bumetanide/amiloride for periods of up to one year. In the 60 to 74-year-olds, mean serum sodium levels decreased from 141.6 to 140.6 mmol/L (P < 0.05), and mean potassium levels increased from 4.3 to 4.5 mmol/L (P < 0.001). In patients 75 years of age and older, mean sodium levels decreased from 140.9 to 139.8 mmol/L (P < 0.01), and mean potassium levels increased from 4.3 to 4.5 mmol/L (NS). Changes in electrolytes during treatment with bumetanide/amiloride were most apparent during the first 12 weeks of treatment. Serum sodium levels below 130 mmol/L and serum potassium levels above 5.6 mmol/L were not recorded. Serum urea, creatinine, and uric acid increased in both age groups, increases in urea being greater in the older patients (P < 0.02).