To investigate the mechanism of atrial flutter (AF) in humans, we studied 13 patients during episodes of spontaneous common AF, with simultaneous multiple atrial endocavitary recordings and atrial programmed stimulation. In all patients, low paraseptal atrial activation preceded high right atrial activation, and the latter preceded mid- or low lateral right atrial activation (recorded in five patients). Programmed atrial stimulation resulted in early reset of the AF cycle, with an unchanged poststimulation AF activation pattern. The poststimulation cycle recorded from an even potential to the site of stimulation was always shorter than the basic flutter cycle length. The poststimulation cycle recorded at the site of stimulation was always equal to or longer than the flutter cycle length. These results strongly favor the existence of a reentry circuit to which the extrastimulus has access.