We studied the relationship between the repetitive (two or more) extrasystole threshold (RET) and ventricular fibrillation threshold (VFT) in 38 pentobarbital anesthetized dogs. A 100-Hz train of 16 4-msec stimuli was delivered to the right ventricle during the T wave of every 12th paced supraventricular beat. Current was increased in 1-mA steps until ventricular fibrillation occurred. Five measurements were made in each dog at 15-minute intervals. In 10.3% of the VFT determinations, VF was not preceded by a repetitive extrasystolic activity. For the remainder regression analysis of the correlation between RET and VFT revealed an r = 0.37 (P< 0.001). The average RET was 54.6% of the VFT (5.4 ± 1.6 vs. 10.2 ± 2.4 mA); however, it ranged from 5% to 98%. Five animals received a 90- minute lidocaine infusion (0.3 mg/kg per min). Although lidocaine significantly increased both RET and VFT as a linear function of log lidocaine plasma concentration (Cp), the mean slope of the VFT vs. log lidocaine Cpregression line was significantly different from that of RET vs. log lidocaine Cpregression line (P< 0.05). Nine dogs infused with the /S-blocker acebutolol (0.5 mg/kg in five and 2 mg/ kg in four dogs) showed a concentration-dependent increase of VFT without significant increase of RET, and the mean slopes of the response vs. log Cpregression lines were significantly different (P<0.01). We conclude that in the control state the RET may be related to the VFT. However, during pharmacological studies, VFT and RET do not necessarily change in parallel. Circ Res 46:599-605,1980