Background The mechanisms of common atrial flutter entrainment have not been directly studied in humans.Methods and Results Endocardial mapping in six cases of common flutter showed large right atrial (RA) reentry circuits.Activation was craniocaudal in the anterolateral right atrium and caudocranial in the septum. The inferior vena cava-tricuspid isthmus (IVC-TV) closed the circuit. The high right atrium was paced at progressively shorter cycle lengths (CLs) in all, and the IVC-TV was paced in three cases. We recorded six to eight simultaneous RA electrograms from septum and anterior wall. Transient entrainment was recognized from all sites by capture of all electrograms at two or more paced CLs, with total or partial preservation of baseline flutter sequence and return to baseline after pacing. Antidromic circuit penetration was documented in five cases during high RA pacing and in one with IVC-TV pacing. Short CLs induced orthodromic conduction delays that resulted in a postpacing pause longer than basal flutter CL. ECG fusion with high RA pacing correlated poorly with antidromic septal penetration. This was related to overlap of orthodromic septal activation with anterior wall activation of the following cycle. Pacing disorganized flutter into a brief irregular rapid rhythm in two cases and atrial fibrillation in one case. In two cases, complete antidromic septal penetration led to sudden flutter interruption, and in another case it led to circuit inversion.Conclusions Direct recordings confirm orthodromic and antidromic penetration of flutter circuits by high and low RA pacing.Short CLs modify the circuit. Disorganization is the most common mode of flutter interruption. (Circulation. 1994;89:2117-2125.)