OBJECTIVE--To investigate whether angiotensin converting enzyme inhibitors reduce diuretic induced magnesium excretion in patients in congestive cardiac failure. DESIGN--Cohort analytic study. SETTING--A London district general hospital. SUBJECTS--Thirty four patients with chronic congestive cardiac failure caused by ischaemic heart disease or cardiomyopathy selected consecutively from inpatients under the care of two consultant cardiologists. Nineteen patients (group 1) on diuretics alone were compared with 15 patients (group 2) taking diuretics plus either enalapril or captopril. All drug regimens were stable for at least three months before the study. Patients with impaired renal function (plasma creatinine greater than 120 mumol/l) were excluded. INTERVENTIONS--An intravenous loading dose of magnesium sulphate was given to minimise the variability in baseline magnesium state. MAIN OUTCOME MEASURE--Total urine magnesium excretion and creatinine clearance in 24 hour urine collections. RESULTS--Plasma magnesium was similar in the two groups. However, 24 hour urine magnesium excretion was significantly lower in group 2 than in group 1. Furthermore, creatinine clearance was also significantly lower in group 2 and correlated strongly with magnesium excretion. There was no such relation in group 1. There was no difference in fractional clearance of magnesium between groups. CONCLUSION--Angiotensin converting enzyme inhibitors have an important magnesium conserving action, possibly via their effect on glomerular filtration rate.