Objective: To assess left ventricular systolic synchronicity and function with right ventricular out tract(RVOT) pacing or right ventricular apex(RVA) pacing by real-time three-dimensional echocardiography(RT-3DE). Methods:In the acute tests of anesthetized closed-chest beagle canines,acustom designed bipolar screw-in lead was implanted using a steerable delivery catheter guided with X-ray. The bipolar leads were implanted in the right ventricular apex or the right ventricular out tract. The dogs received right ventricular pacing at the rate of(138±33) beats per minute. SIEMENS ACUSON SC2000 with 4Z1c probe was used to perform full volume RT-3DE,16-segmental volume curves were obtained by the on-line LVA software.The end-systolic volume(ESV),end-diastolic volume(EDV),Stroke volume(SV),left ventricular ejection fraction(LVEF),the standard deviation(SD)and maximal difference(Dif) of the time to point with minimal systolic volume(Tmsv)of 16 segments,12 segments(6 mid and 6 basal)and 6 segments(basal)were calculated.Tmsv-SD% and Tmsv-Dif% were used to eliminate the influence of different heart rate. Results:There was no significant difference between sinus rhythm(SR)group and RVOT group(P > 0.05).After RVA pacing,Tmsv-16SD%,Tmsv-12SD%,Tmsv-6SD%,Tmsv-16Dif%,Tmsv-12Dif% and Tmsv-6Dif% were significant worsen than those in SR group(P < 0.01),while LVEF was lower(P < 0.05). Compared with RVOT group,Tmsv-16SD%,Tmsv-12SD%,Tmsv-6SD%,Tmsv-16Dif%,Tmsv-12Dif% and Tmsv-6Dif% were increased,and LVEF was decreased significantly(P < 0.05)in RVA group. Conclusion:RT-3DE is a feasible and reproducible method to quantify left ventricular synchrony and function.RVOT pacing in dogs is associated with more favorable immediate myocardial contraction and mechanical synchrony compared with RVA pacing.