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In a group of 152 elderly patients (age range between 65 to 92 years) and 54 patients (aged 35 to 64 years) plasma digoxin level measuring 2.5-6.2 nmol/1 were correlated with the clinical symptoms and the electrocardiogram. The conduction disturbances and arrhythmias as well as the P-R interval, P-T-Q index and corrected Q-T interval in the ECG were analyzed. The clinical symptoms of hypersaturation with digitalis were present in 54.6% of the elderly and 35.2% of the younger patients. Conduction disturbances were found in 42% of the elderly and 22.2% of the younger patients, while arrhythmias appeared in 40.1% of the elderly and 31.5% of the younger ones. 17.8% of the elderly and 46.3% of the younger patients were without these changes. The correlation between P-R interval and high plasma digoxin level in the elderly (p < 0.01) and younger patients (p < 0.05), as well as between the P-T-Q index and high plasma digoxin level in the elderly (p < 0.01) was found. There was no correlation between the corrected Q-T interval and high plasma digoxin level in both groups. No correlation was found between a high plasma digoxin level and serum creatinine level in both groups, neither between a high plasma digoxin level and serum potassium level in both groups. The effect of digitalis has not been shown to be a cause of specific changes in an electrocardiogram neither in the elderly nor in younger patients. However, the association between prolonged P-R interval as well as changes in the P-T-Q index and high plasma digoxin level has been found more often in the elderly than in the younger patients.