We investigated the age-specific arrhythmogenic effects of digoxin in the rat. Adult female rats (n = 26) and one-day-old newborns (n = 20) were anesthetized with pentobarbital and injected subcutaneously with varying doses of digoxin. Electrocardiograms (ECG) were monitored continuously for four and one-half hours following digoxin administration. The arrhythmogenic dose 50 (AD50) and lethal dose 50 under anesthesia (LD50) were determined using the method of Litchfield and Wilcoxon. AD50 in adults was 13.0 +/- 1.0 mg/kg (X +/- SD) compared with 2.9 +/- 0.3 mg/kg in the newborns (P less than 0.01), and LD50 in adults was 30.0 +/- 1.9 mg/kg compared with 5.0 +/- 0.2 mg/kg in the newborns (P less than 0.01). Arrhythmias appeared earlier in adults (54 +/- 11.5 minutes after digoxin, X +/- SEM) than in newborns (132.2 +/- 11.0 minutes, P less than 0.01). Paroxysmal atrial tachycardia was the predominant arrhythmia in adults (73%), while transient sinus bradycardia appeared in only 9%. In contrast, all newborns with arrhythmias had severe sinus bradycardia and 69% had profound first degree heart block as well. In conclusion, the newborn rat is more sensitive to digoxin toxicity and develops lethal arrhythmias much more readily than the adult.